A few questions.
How are you trying to open the app in Studio Pro, are you clicking on Edit in Studio Pro in the developer portal or are you starting Studio Pro and then clicking on Open App at the top of the screen or Open on the right side of one of the apps?
Also, what version of Studio Pro do you have installed?
If you’ve just created a new app in Studio, then make sure you have the latest version of Studio Pro installed 9.1.1 as when you create a new app in Studio it will automatically select the latest modeller version which is currently 9.1.1.
Do you get the version not found screen pop up on the Mendix Version Selector?
First publish it in Studio, making sure it contains no errors. Then try again to open the app in Studio Pro.