When downgrading your pricing plan to the START plan, a situation has occurred where you have more than 10 required fields.
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.