Problems transferring music files from iTunes ver. 12.9.4.94 MacBook Pro (running macOS Mojave v. 10.14.4

Suddenly, today when I drag and drop an mp3 file from iTunes’ song list into 'Devices', i.e. my iPhone 8+, I get a message I've never seen before:

"Some of the files were not copied to the iPhone "Peter's iPhone" because iCloud Music Library is enabled on this phone."

Can someone please explain exactly what this means, please? Do I have to "disable iCloud Music Library on my phone?" If so, how do I do that?

Any solutions, suggestions re this irritating problem would be most welcome.