smc

First, modulo $2$ or $5$, $\underline{20210A}\equiv A$. Hence, $A\ne 0,2,4,5,6,8$. Second modulo $3$, $\underline{20210A}\equiv 2+0+2+1+0+A\equiv 5+A$. Hence, $A\ne 1,4,7$. Third, modulo $11$, $\underline{20210A}\equiv A+1+0-0-2-2\equiv A-3$. Hence, $A\ne 3$. Therefore, the answer is $\boxed{9}$.