When downgrading your pricing plan, a situation occurred where you have more required fields than the maximum limit of the newly requested plan.
The START plan does not support required fields (all existing required fields must be removed)
The PROFESSIONAL plan has a limit of 25 required fields
The ULTIMATE plan has no limit
This problem can be solved by removing the Required field flag from fields that don't need to be required. This will bring you to the set out maximum number of required fields.
The required field flag can be removed by a CRM account administrator. Click on your name » Settings » Record Settings. Here you need to select a record type, click on Field Settings and select the field with the Required field flag and disable this flag.
A detailed article on required fields can be found HERE.