We don’t recommend using the Sandbox to test the Add Team Member flow. The Sandbox doesn’t fully support all Production Dashboard features, including this Add Team Member flow, because they rely on modules that aren’t available in the Sandbox environment. That is what’s causing the errors you’re seeing.
Here are two alternatives:
Add Team Members on API Explorer in Sandbox mode You can create team members with the Team API’s CreateTeamMember endpoint.
OR Test end to end in Production mode You can use either your existing account or a separate developer account to test this flow in production.