Hi,
Are you trying to check mark Enable Assisted Company Setup field in companies page?
if yes this field is used to give you assisted company setup feature which will ensures that all the required setups are completed and the users can start using the system.
Please note: You cannot turn it on for the company whose Evaluation company is checked because it will throw you the validation error.

Also without Evaluation company check marked for the companies you can mark Enable Assisted setup.

If my answer was helpful to you, please verify it so that other users know it worked. Thank you very much