If you are a new user and you are unable to log in after signing up, please take a look at this article: Resolving the sign up issues
Also, make sure to check if the Office365 account you are using to log in to Matter365 is the same one that you are logged on with the Office365 on your device. If you are logged into a different Office 365 account on your computer, it may result in an error.
Matter365 automatically tries to log you in with the account you have on your device.
If you are using different Office365 accounts, you must open a Private/Incognito window in your browser and log in there.