See the documentation here: https://docs.mendix.com/refguide9/version-control/#studio-enabled
It is about this part:
Studio cannot be used to develop the app if it is not enabled for any development lines.
Regards,
Ronald
Although @Ronald was correct in referring to the relevant documentation, the important part here is that you started the application in Mendix 9.21, which also happens to be the version that no longer support collaborative development using Mendix Studio.
You have two choices, only of of which is future-proof:
- Start again in a lower version of Mendix to use both Studio Pro and Studio (not preferred)
- Stop using Mendix Studio (preferred)
```Mendix Studio is no longer supported for apps with Mendix version 9.21 or above.```