MacRumors has been able to reproduce this issue in iTunes version 188.8.131.52 on macOS Mojave version 10.14.3, although based on comments across the MacRumors forums and Apple Support Communities, the error message does not appear to be limited to specific versions of iTunes or macOS or any other parameters.
Many users are able to access the "For You" tab as usual, but others have been seeing this error message persist since early February. Affected users are still able to access "For You" on an iPhone, iPad, or other device with Apple Music.
While some affected users can temporarily access the "For You" tab in iTunes after signing out and back in to their Apple ID account, the error message eventually returns. On our affected machine, nothing we have done has restored access to the tab, including force quitting iTunes and restarting the Mac.
All in all, this is a minor issue, but hopefully one that is addressed soon. We reached out to Apple to see if it is aware of the issue, but we have yet to hear back. One affected user claimed that Apple has engineers working on a fix.