NonParallelPlanReason="CouldNotGenerateValidParallelPlan" - one of the possible reasons

When you see your query doesn't go parallel and in the execution
plan XML you see NonParallelPlanReason="CouldNotGenerateValidParallelPlan".
Until SQL Server 2022 there is no indication why and what is the reason.

One of the possible reasons can be that you are querying a table which has
scalar function (UDF) in a check constraint or in a computed column.

Scalar UDFs in check constraints and in computed columns prevent parallelism (even for  SELECT Queries)

Try to disable the check constrain and check if it goes parallel.