So by now you are probably pulling your hair over this error. If you want to lose all your hair try talking to someone at Google support about the issue, you will lose your mind and hair!!!
Basically Google Play Music allowed you upload your music within the Chrome browser if you installed the Google Play Music app/extension to the Chrome browser. But then starting sometime in late Nov 2018 you started getting that error.
When I contacted Google about the problem, it started in early, the support person insisted it was never possible to drag your music to the browser to upload music! Yep I got a typical winner from Google. After telling her multiple times it’s been working for years she finally decided to change her tune and listen.
I told her I had multiple accounts and computers and it was obvious it was the new Chrome browser. I had a laptop that I don’t use often that had an older version of Chrome installed. I was able to upload music with no issues on the laptop. Then I selected the function to upgrade the browser to Chrome v71 and poof, I no longer could upload music.
So the solution is to downgrade the browser to v70. But Google does not allow you to download/install older versions. Luckily using Google search I found a site that houses older versions of Chrome. First you need to uninstall v71 of Chrome or the older version will not install. Make sure you leave your browsing history intact during the uninstall. Then I installed v70.0.3538.77 from a site called slimjet.com The exact URL is listed below.
I highly recommend keeping this older copy on your computer, who knows Google could shutdown that site. Also don’t look at the current version installed, that will trigger the auto-update and install the latest BUSTED v71 which won’t play nice with Google Play Music.
Lastly the support person told me I had to provide feedback from GPM. She alone can’t really open a ticket proving that it does not work! She can’t even see the feedback on the forums or Google reviews on the app not working since v71. No wonder Google takes so long to fix bugs, their entire support structure is retarded. I know this from several whooping Pixel/Android bugs. Google takes months if not years to fix issues.