Hello,
I am trying to run some regressions and getting an error that says there are no observations, even though I believe there should be 51. I am aware this is a small sample size, but I am just trying to get a feel for the relationship between my two main variables: lopcold100 and lpop2000d100. My code is bellow

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input str53 state byte fips_state_code float(land2 alti malaria) double rainfall float(lpopcold100 lpop2000d100)
"Alabama"              26 .400114  .075 .10021    1.4551025390625  -5.653392 -.14538972
"Alaska"               55  .00684     .      0                  .          .  -5.449607
"Arizona"              18 .260931  .342 .22284 .27264358520507814    -5.5732  -.6962354
"Arkansas"             39 .511114  .091 .09995 1.3906500244140625  -6.996193  -.6633876
"California"           17 .461956  .004 .00063     .4063662109375  -3.895112   .7955512
"Colorado"             28 .361079 1.613      0 .29992318725585937  -7.222957  -.8244287
"Connecticut"          21 .377271  .013 .00853 1.0896600341796876 -4.5318146  1.8295695
"Delaware"              1 .390473  .005 .02202 1.0373360595703125 -4.6701636  1.2298617
"District of Columbia" 47    .329     . .03404                  .          .   3.488944
"Florida"              42 .352214  .053 .10632    1.3417841796875  -6.534948  1.0426546
"Georgia"              36 .322205  .304 .09592   1.22775146484375  -5.515173    .400922
"Hawaii"               34       .     .      0                  .          .  -.8230577
"Idaho"                56 .460358  .834      0  .3105574035644531   -7.04038  -1.799212
"Illinois"             16 .855963  .182 .04295   .928014404296875  -6.220318   .7772537
"Indiana"              35 .890171  .221  .0329   .996759521484375  -6.661077  .53155136
"Iowa"                  8  .88738  .267 .02767    .83210400390625  -6.163188  -.6467861
"Kansas"                4 .859178  .273 .05381  .6757924194335938  -7.250305 -1.1037669
"Kentucky"             30  .61795  .151 .05147    1.1932919921875  -8.374496  .02117748
"Louisiana"            49  .66261  .014 .10357 1.4206219482421876  -5.497486 -.11224628
"Maine"                32 .203591   .02 .00021   1.02793798828125  -6.807815  -.9557078
"Maryland"             23 .416147     0 .03404  1.035303955078125  -7.225884  1.4927202
"Massachusetts"        33 .416991  .002 .00442  1.170177978515625  -3.829518  1.9384042
"Michigan"             25 .448933  .257 .00356  .8148601684570312  -6.608004 .033505138
"Minnesota"            44 .713185  .228 .00546  .7100316162109375  -6.014863  -.5433291
"Mississippi"           9 .566087   .08 .10718       1.4219765625  -4.827112 -.51756936
"Missouri"             50 .672554  .181   .063  .9983469848632812  -8.632043 -.19696847
"Montana"               2 .611333 1.262 .00006  .3506045837402344  -6.733862  -2.766566
"Nebraska"             53 .765595   .69 .02014  .6043930053710938  -6.126088 -1.4932418
"Nevada"               41 .280016 1.462      0 .20294599914550782  -7.445746  -1.587216
"New Hampshire"        15 .255507  .084 .00112 1.7186910400390625  -5.399504   .3191277
"New Jersey"            6 .350681  .015 .02371   1.07010205078125 -4.6701636  2.3432877
"New Mexico"           45 .240202 2.152 .34678  .3094565734863281    -5.5732  -1.866243
"New York"             11 .505778   .02 .00454 1.0150404052734374  -5.912641   1.268715
"North Carolina"       37  .34166   .09 .05668 1.2068809814453125   -6.64705   .4773803
"North Dakota"         10 .866063  .516 .00294  .4113954162597656  -6.632054 -2.4057255
"Ohio"                 24 .780372  .234 .01894  .9507855224609375  -6.743628   .9333655
"Oklahoma"             12 .838393  .359 .09895  .9391649780273438  -6.016211  -.6876569
"Oregon"               51 .607906  .058 .00002  .8501062622070312  -5.121648 -1.0022193
"Pennsylvania"         13 .439329  .108 .01062 1.0198417358398437  -6.608177    .982283
"Rhode Island"         54 .387793  .002 .01157 1.1564620361328124  -3.829518   2.151406
"South Carolina"       38 .330103  .084 .08542     1.277810546875  -7.239993  .29349166
"South Dakota"         31 .927367  .442 .01264  .5024754943847656  -6.281605 -2.3015249
"Tennessee"            27 .511743  .138 .06338 1.2466744384765625  -8.416538   .3349522
"Texas"                20 .649587   .14  .5335  .7368400268554688  -6.467943 -.18560813
"Utah"                 46 .397048 1.308      0    .32943798828125  -7.379053   -1.25411
"Vermont"              19 .339573  .182 .00023  .8755380249023438  -6.332584  -.4459432
"Virginia"             29 .410387  .048 .04094  1.065479248046875  -5.871078   .5560539
"Washington"            5 .467992  .027 .00006 1.0812779541015625  -4.822076  -.1385782
"West Virginia"        48 .486838  .174 .01722  1.078801513671875  -5.871078 -.29412043
"Wisconsin"            40 .667363  .267 .00529  .7822564697265625  -5.584909   -.178485
"Wyoming"              22 .372659 1.856      0  .3455035095214844  -6.804948  -2.965019
end
Here is the code that I have been running:
Code:
global contws2 "land2 alti malaria rainfall"
qui eststo m1: reg   lpop2000d100  lpopcold100                ,robust cluster(fips_state_code)
qui eststo m4: xtreg lpop2000d100  lpopcold100 ${contws2}    ,fe robust cluster(fips_state_code)

esttab m1 m4,star(* 0.1 ** 0.05 *** 0.01)  b(a1) se(2)  ///
stats(N r2_a, fmt(0 3) labels("N" "R^2")) order(lpopcold100 $contrws2) ///
title("Table 5: Persistence in Population Density (pooled)") ///
mtitle ("OLS" "Between" "Within FE" "Within FE" ) compress longtable nogaps nonumber ///
replace nonotes coeflabels(lpopcold100 "PreColonialDens" land2 "Agriculture" ///
alti "Altitude" rainfall "Rainfall" malaria "Malaria" _cons "Constant")  varwidth(15)
eststo clear
The first line of regression runs fine, but I get an error saying no observations when I run the second regression line. (Line 3 above). Any idea where I am going wrong?