When working with SQL Source Control Projects, SQL Change Automation Powershell requires the same permissions as SQL Compare: https://documentation.red-gate.com/sc14/getting-started/requirements/permissions-required-to-use-sql-compare

Build step specific:

Besides the permissions listed above please note that the following permissions might be required when doing the schema validation:

-  sysadmin if using the Temporary Server option (because when it creates the Database it sets TRUSTWORTHY ON).

- db_owner if using the Temporary Database option.