How and where to ask for support
There are 2 sites, musescore.org and musescore.com: the same login works for both, and they share a common Help Center.
musescore.org is the site for questions about the MuseScore editor software itself, the free, open source desktop software (Windows, Mac, Linux...) and its support and documentation.
(.org is the top level domain for non-profit organizations)
musescore.com is the site for posting, sharing, downloading, and discussing MuseScore scores; It is also for discussing the mobile apps (for iOS, Android, Fire OS). You can also import music PDFs, and send scores to YouTube. It is here where you can have a "Pro" (paid) or limited "free" account.
(.com is the top level domain for commercial sites)
Support for Muse Hub is offered by their team on their platform: https://musehub.zendesk.com/
For musescore.org there are several forums divided by categories:
While for musescore.com there are numerous discussion groups
Like the ones dedicated to the MuseScore.com website, Improving MuseScore com and MuseScore Updates and Status and also
those dedicated to the Mobile Apps, like MuseScore on Android and MuseScore on iOS
Any questions or issues concerning billing (of a Pro subscription) need to be sent to email@example.com or the "Contact us" on the footer of the musescore.com website (and the "Request a refund" button there). This address/contact is not monitored 7*24, but only 5*9, during regular business hours in Limassol, Zyprus, i.e. the Eastern European Timezone, so please allow up to 2 business days for a response.
There is no billing support via public discussions, neither in the forums on musescore.org, nor in groups on musescore.com, the staff responsible for billing simply don't read either and posting here or there only annoys other users.
See also Request a Refund.
To cancel a Pro account/subscription go to your profile
To report a bug on .com go to MuseScore.com Bug-Tracking Form