MacRumors has reproduced this issue in iTunes version 126.96.36.199 on MacOS Mojave version 10.14.3, but based on comments on MacRumors forums and Apple Support Communities, the error message does not appear to be limited to specific versions of iTunes or MacOS or other parameters.
Many users can access the "For You" category as usual, but others have seen this error persistently since the beginning of February. Affected users can still access "For You" on an iPhone, iPad or other device with Apple Music .
While some affected users may temporarily access the "For You" tab in iTunes after logging out and returning to their Apple ID account, the error message finally comes back. On our affected machine, nothing we've done has restored access to the tab, including power outages in iTunes and restarting the Mac.
All in all, this is a minor problem, but hopefully one will be dealt with soon. We traveled to Apple to see if it is aware of the problem, but we have yet to hear. An affected user claimed that Apple has engineers working on a solution.