Hi afje2x3zj, welcome to Thunkable! 
Be sure to check out How to ask Great Questions v2.0, the Community Guidelines, and our Getting Started Guide to make the best of your Thunkable Community experience!
This popup is shown because you likely exceeded the limit of active published apps available for your plan. If you switched from the Pro plan to the Builder plan, you will need to ensure that you only have one active published app.
You can find more information on how to set your active published app in our Docs: