When downgrading your pricing plan to the START plan, a situation has occurred where you have more than 3 trade types.
This problem can be solved by deleting all other trade types and keeping only 3, which is the set out maximum number of trade types in the START 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.