Can I sign in with Google if I already created an account with email and password?
Yes! You can link your account to Google provided the email you used to sign up is a Gmail address.
I signed up with Google and am unable to perform some operations on the app. How can I fix this?
Some operations require you to set a password, like
Change your email
Delete your account
To do so, click the Set Password button in the User Account settings (which only shows up if you haven’t set a password before) or use the reset password page.