All versions of Windows 10 after 20H2 show up as version 2009 in Resource Manager as seen here:
ITMS 8.x
This is resolved by applying a point fix for the version being used and these are documented in the following:
Windows 10, version 21H2 Support Statement
Last Updated March 11, 2022
Symantec IT Management Suite versions 8.5 RU4, 8.6 RU1, and 8.6 RU2 support the management of Windows 10, version 21H2 client computers.
Support for Windows 10 21H2 is natively supported in ITMS 8.6 RU2, and in ITMS 8.6 RU1 and 8.5 RU4 once the following point fixes have been applied:
8.5 RU4 - SMA_SMP_8_5_POST_RU4_v9_22Nov.zip (download from 198337)
8.6 RU1 - SMA_SMP_8_6_POST_RU1_v6_23Nov.zip (download from 221269)
198337 CUMULATIVE POST ITMS 8.5 RU4 POINT FIXES
221269 CUMULATIVE POST ITMS 8.6 RU1 POINT FIXES