我的问题是：year变量原先是红色，格式是“2001-12-31”，我用encode 已经使其变蓝色了，现在回归要用，下面我试了好几次的过程，请哪位诊断诊断。

. xtreg cost tyconverg roa herfindal3 zczz ldblv lntass mb1 xihi1 tobin1 lever1 tasset glzf1 if year1==2003,fe
insufficient observations
r(2001);

. xtreg cost tyconverg roa herfindal3 zczz ldblv lntass mb1 xihi1 tobin1 lever1 tasset glzf1 if year1==2003-12-31,fe
insufficient observations
r(2001);

. xtreg cost tyconverg roa herfindal3 zczz ldblv lntass mb1 xihi1 tobin1 lever1 tasset glzf1 if year==2003-12-31,fe
type mismatch
r(109);

. xtreg cost tyconverg roa herfindal3 zczz ldblv lntass mb1 xihi1 tobin1 lever1 tasset glzf1 if year==2003, fe
type mismatch
r(109);

. xtreg cost tyconverg roa herfindal3 zczz ldblv lntass mb1 xihi1 tobin1 lever1 tasset glzf1 if year=2003, fe
type mismatch
r(109);

. xtreg cost tyconverg roa herfindal3 zczz ldblv lntass mb1 xihi1 tobin1 lever1 tasset glzf1 if year1=2003, fe
invalid syntax
r(198);