Further to this, we checked in our backend logs and confirmed that there are custom fields set on your client page. We assume that these must have been created after the client was created as there exists no values in these fields for this client. May we please request you to check these fields and fill in the required information. Once done you may proceed with changing the client status.
Note - The above explanation holds good as the custom fields are required.
Please let us know if the issue differs our understanding.