Here are some tips that may help if you're having trouble logging into the mobile app:
It's easy to make a typo on a mobile keyboard. Triple-check that you entered everything correctly.
Where to find your subdomain / account identifier if you don't remember it.
Username and password are case sensitive. If your username is "Fred", you will not be able to login by typing "fred".
Your username is not usually the same as your email address.
Some Android devices will automatically capitalize the first letter of your password. Watch for your keyboard to highlight the caps arrow.
If none of those steps help then the app may need to be updated to the newest version. You can do that from your device's app settings or just uninstall/reinstall the app.
Keep in mind that if your device's Operating System (Android or IOS) is outdated then you will get a login error as if your username/password is incorrect.