You may have the testing server box ticked, but the error is telling you that the testing server settings are not correct.
It is telling you that the issue is with the URL Prefix you have specified for the testing server.
Are you setting the testing server to be a remote testing server or a local testing server?
Many hosts will have issues if you are using the remote server as the testing server.
it is recommended to use a local testing server. the Learning resources page has information on installing a local testing server:
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