When downgrading your pricing plan, a situation occurred where you have more deal types than the maximum limit of the newly requested plan.
The START plan does not support the Multiple Trade Types extension (all deal types must be removed)
The PROFESSIONAL plan has a limit of 5 trade types
The ULTIMATE plan has no limit
This situation can be resolved by removing existing deal types down to the required number. This will bring you to the maximum number of deal types allowed by the selected plan.
Decide which trade type you wish to delete. You can find an overview of the used trade types, e.g., on the Business Board or in Record Settings » Business. First you need to transfer all open deals falling under this trade type to another trade type or close them (Won, Lost, or Canceled deal stage).
Then, if you are an administrator, click on your name » Settings » Record settings » Business » Deal » General settings » Trade types » Edit settings and delete the specific trade type by clicking on the X.
A detailed article on trade types can be found HERE.