Did you manage to resolve this issue? I'm having same problem on Big Sur. Documentation seem to be wrong because it states that this API should work on Big Sur: https://developer.apple.com/documentation/security/1617986-secaddsharedwebcredential?language=objc
After checking source code of "Security" project it looks like it is not fully implemented: https://opensource.apple.com/source/Security/Security-59754.80.3/OSX/sec/Security/SecSharedCredential.c.auto.html
// OSX now has SWC enabled, but cannot link SharedWebCredentials framework: rdar://59958701
How can we track this rdar?