You dont need the file extension in the path (.mp3)…
Other than that, you need to use the pipeline tool, to build the mp3 file into a pair of files: a songName.wma and songName.xnb, and then include THOSE files instead.
It can seem like a hassle, but once it works, it works…
I changed the file from mp3 to a wma, and the same error is still there. I looked through the project files in file explorer and the file is converted to xnb with the wma version, but it supposedly still can’t find it.
Go to the /bin directory of your project and look if both will.xnb & will.wma files are there.
I mean the directory where your .executable is, it should be “Bin/Debug/Content” “Bin/Release/Content” or something similar.
If you create a project from templates it has a /content folder and a content.mgcb file with it’s action is set to ‘MonoGameContentReference’.
Any asset you add to that .mgcb will be automatically copied to the output directory.
There are other ways that require editing your project file or manually coping assets and include them as resources, but the above is the is easiest method. Do you have this in your project? If you used some old sample as your starting point you might not. It’s easy to to add it, Let us know what’s the situation and how you currently include that .mp3.