Creating SQL Assembly error Method parent has circular class type parameter constraints
The issue probably has something to do with the BigInteger class. It is found in System.Numerics which is not in the list of Supported .NET Framework Libraries. Which means that you probably loaded that manually as UNSAFE (or have not loaded it yet?). Which then means that your assembly, HMDACalculateULICheckDigit, needs to be marked as UNSAFE.

This assessment is further supported by the fact that you got a "failed verification" error message, since verification is a process that happens at CREATE ASSEMBLY time and checks for obvious indicators of requiring UNSAFE.