Hi, I am using almost the same code to count TFP, but it continues to saying" missing values in cluster variable __000002 not allowed r(459);". The first time I run the code, it went well. And days later when I tried, it failed. I tried to change the code several times but it still can work. what does it means? How can I fix?

the code is as follows
opreg lnY_add, exit(exit) state(age lnK) proxy(lnI) free(lnL imp exp) ///
cvars(year) vce(bootstrap, seed(1357) rep(10))

previous code is
opreg lnY_add, exit(exit) state(age lnK) proxy(lnI) free(lnL) ///
cvars(SOE) vce(bootstrap, seed(1357) rep(5))

below is part of the data
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Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input float(lnY_add exit age lnK lnI lnL) byte(imp exp) int year
 9.785761 0  9 10.110442 8.5277195 5.252274 1 0 2001
 9.593349 0 10 10.262786         . 5.198497 1 0 2002
 8.200395 1 11 10.153196         . 5.129899 0 0 2003
 9.313187 0  8         .         . 4.787492 1 0 2001
 9.487337 0  9         .         . 4.941642 1 0 2002
 9.882194 0 10         .         . 4.787492 1 0 2003
10.586654 0  6         .         .        . 0 0 2004
11.083817 0  7         .         . 5.036952 0 0 2005
10.954542 0  8         .         . 5.099866 0 0 2006
10.841356 0 34         .         . 9.242227 1 0 2001
end
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