Bugs happen, it’s a way of life but when the bug affects you as a developer, it can be a tad annoying. I’ve found a bug and mentioned it in the MonoGame forums.
The bug occurs intermittently. I do a build then run the program on an Android phone. It starts loading, shows the splash screen then splat “The Content file was not found” message appears in Visual Studio.
This may be a bug in Xamarin or MonoGame. No doubt it will eventually get fixed but still a bit annoying. About the main workround I’ve found is rebuild everything. Start with the Pipeline tool- do a Build/Clean then a Build/Build. Then in the Project (in Visual Studio) do a Build/Clean then Build/Deploy which recompiles everything.
If the error still persists then it’s possible you have a typo; say a lowercase i instead of I in a filename. (I’ve done that once). Another fix though rather unsatisfactory is in the Project Properties, on the Options tab, right at the top is a checkbox Use Shared Runtime. If you untick that, deployment takes a bit longer. Instead of copying 3 MB which takes a second or so, it copies about 60 MB instead which takes a bit longer. I tried it but went back to the Clean and Deploy approach as copying was taking 10-15 seconds.