You will not be able to switch to the Starter version if you have features activated that are only available in the Pro version. Check your account to see if you have:
- More than one user account.
- More than one booking engine.
- Any booking channels activated.
- The “Storing Card Details Securely” feature activated.
- Categorizing room types activated.
If you remove or inactivate all of these, you should be able to switch subscription plan.