I am trying to create lagged "Total Assets" and went through few of the forum posts and instruction to tsset or xtset my data first. Since my data is panel data I used xtset on Year variable but running into error.

The exact steps is used are:

xtset Year >> gen Lagasset = l1.TotalAssets

Any help would be appreciated.

Code:
* Example generated by -dataex-. For more info, type help dataex
clear
input str9 Ticker int Year long TotalAssets
"TH:2S"  1993        .
"TH:2S"  1994        .
"TH:2S"  1995        .
"TH:2S"  1996        .
"TH:2S"  1997        .
"TH:2S"  1998        .
"TH:2S"  1999        .
"TH:2S"  2000        .
"TH:2S"  2001        .
"TH:2S"  2002        .
"TH:2S"  2003        .
"TH:2S"  2004        .
"TH:2S"  2005        .
"TH:2S"  2006   626450
"TH:2S"  2007   739620
"TH:2S"  2008   559340
"TH:2S"  2009   913418
"TH:2S"  2010  1024697
"TH:2S"  2011  1059772
"TH:2S"  2012  1253012
"TH:2S"  2013  1219920
"TH:2S"  2014  1205561
"TH:7UP" 1993        .
"TH:7UP" 1994        .
"TH:7UP" 1995        .
"TH:7UP" 1996        .
"TH:7UP" 1997        .
"TH:7UP" 1998        .
"TH:7UP" 1999        .
"TH:7UP" 2000        .
"TH:7UP" 2001  1239837
"TH:7UP" 2002  1445609
"TH:7UP" 2003  1589719
"TH:7UP" 2004  2561613
"TH:7UP" 2005  3659381
"TH:7UP" 2006  4047774
"TH:7UP" 2007  3776231
"TH:7UP" 2008  4178294
"TH:7UP" 2009  4126859
"TH:7UP" 2010  4403855
"TH:7UP" 2011  4072216
"TH:7UP" 2012  1211540
"TH:7UP" 2013  1229760
"TH:7UP" 2014   922593