Strange... SamMobile firmware are showing it for manual download and installation for the S21, G991B, model on BTU...
You could try going in to Airplane mode and rebooting your phone and then coming out of Airplane mode and manually checking for a software update. Some users have reported this has worked for them.
I have unlocked version in Czech Republic(XEZ) and I also didn't received March update.
I check fota server (Samsung) update server and at the moment latest software for S21 Regural CSC BTU is AUB8
Software is only available for download from unofficial tools strength