That is not a problem of version incompatibility: MATLAB is saying that the file is the wrong format to be a Simulink model.
Sometimes there can be problems accessing files across network drives (especially if the file is on OneDrive) that can be solved by copying the file to a local drive. Sometimes network drives corrupt a file and make it unusable. Sometimes there are permissions problems accessing a file.
And sometimes the file just is not a Simulink model. For example a mdl file might have gotten renamed as slx, or a slx might have gotten unzipped.
You can copy the slx file and attempt to unzip it. A valid slx file will unzip into a directory with a couple of subdirectories, with most of the files being XML files. If the file will not unzip then it is not a valid slx file.
