Using version 24.11.17
It’s greyed out and the only way I can change it is thru the reframe button. I saw another similar post that said to use the resample button, which is suppose to be next to the reframe button, but mine does not have that button.
That button was removed. All that you need to do is simply restart Shotcut. I found the UI can get into this state if you add the Reframe filter and then delete it. Restarting remedies it, or you can change the Video Mode (usually better than changing Export > Video).
Not sure how I ended up in that state either. But restarting did fix it, at least in general. However, if I opened up a project where this was a problem, it went back to that state. In case anyone is reading this in the future, my fix is: remove the reframe filter from the output, save, restart, open the project.
The only way it gets and returns to that state is when you have the Reframe filter on Timeline > Output. That is by design. Clicking Export > Video > Reframe adds that filter.
Fixed for next version 25.01