Has the sim been started in safe mode? If so the community content is disabled and it would make sense (and I believe this is the case) that the market place and content manager are also disabled.
Failing that I would exit the sim and log out of all Microsoft services, like the MS Store, the Xbox app, Microsoft account, then restart the PC. Log back on to those services then try the sim again. It may well be worth/necessary logging out of the sim as well.