Hello,
it depends how old your program is, the MbedStudio can working with MbedOS 5.12+ I think, see this.
Any program can not be set as active when the MbedOS version or its state is not valid.
It can be solved with these steps:
- delete all files or folders with old a mbed/mbed-os
- right click on project’s folder and chose
Add Library...and into the first line place the link (GitHub - ARMmbed/mbed-os: Arm Mbed OS is a platform operating system designed for the internet of things) and then you will be asked about what revision (5.15) you want to import via a select box. And then you will able set the program as active one.
If you already have a MbedOS6 imported then visit the Libraries card and press the down arrow what is close to update button (like is in documentation) and you will see an option Check out to version, then it is same as above.
BR, Jan