When 2021 is written in base 21

$2021_{21} = 2⋅21^3 + 2⋅21 + 1$

$2021_{21} = 2(20+1)^3 + 2(20+1) + 1$

$2021_{21} = 2(20^3 + 3⋅20^2 + 3⋅20 + 1) + 2⋅20+2+1$

$2021_{21} = 2⋅20^3 + 6⋅20^2 + 6⋅20 + 2 + 2⋅20+2+1$

$2021_{21} = 2⋅20^3 + 6⋅20^2 + 8⋅20 + 5$

$2021_{21} = 2685_{20}$

$2+6+8+5 = \fbox{21}$

$2021_{21} = 2 \cdot 21^3 + 0 \cdot 21^2 + 2 \cdot 21^1 + 1 \cdot 21^0 = 18522$

$18522 = 2 \cdot 20^3 + 6 \cdot 20^2 + 8 \cdot 20^1 + 5 \cdot 20^0 = 2685_{20}$

$2 + 6 + 8 + 5 = \boxed{21}$ .