When downgrading your pricing plan to START, you may encounter a situation where the Security Levels feature is still active, even though it is not available in this plan. Therefore, the feature must be deactivated.
An administrator can do this by clicking on their name » Settings » Security » Security Levels, where all levels except the system level named Shared must be removed using the delete (cross) icon.
If the system does not allow you to remove a security level, you need to filter the records in list views where the level is used and update the Security Level for those records using the Bulk Edit function.
In each list view, make sure to display inactive records as well.
Once all security levels have been removed, you can proceed with switching to the START pricing plan.