I have got some m4a files from the same source. Some are editable, some are not.
I think the error message is way too generic. Something like "Cannot write tags to file "Foo.m4a" in folder "bar"".
I think it doesn't help to resolve the issue and as a trial is not so promising.
If you have logs, why don't you put a reference to them so that one may try to find a solution?