Microsoft SQL Server 2019 (RTM-GDR) (KB4583458) - 15.0.2080.9 (X64) Nov 6 2020 16:50:01 Copyright (C) 2019 Microsoft Corporation Developer Edition (64-bit) on Windows Server 2019 Datacenter 10.0 <X64> (Build 17763: ) (Hypervisor)
While unlikely to be related to this problem, you should apply the most recent Cumulative Update which is CU13 to get access to many bug fixes in SQL 2019.
Using SSMS, not scripts, database/security/users, select "New User". User type = Windows user, Select a windows user "machinename\username, Login name = username, default schema = dbo, click OK
I tried this and it worked for me with SSMS 18.10. Which version of SSMS do you have? Did you try the Script button to see what SSMS actually generates? It would help to see a screenshot.