Why does it say an account already exists for my email?
When you accept a landlord's invite, PropertyFolio adds it to the account that already uses your email address - but only once you prove that account is yours. If the invite screen says you already have a PropertyFolio account, choose Sign in to accept invite, enter that account's password on the same screen, and the invite is added to it. The invite stays open until you do.
Do not remember that password? Choose Reset password and follow the emailed link. Resetting confirms the email is yours and signs out every other device. Then open your invite link again and sign in to accept it. If you created that account with Google or Apple, use that button on the invite screen instead.
Google or Apple sign-in can also ask you to sign in the way you created the account. That happens when it cannot open the existing account for your email: the account was created with a password, or with a different Google or Apple sign-in, and its email has never been confirmed - or Google or Apple did not confirm your email address, which can happen with a work or school Apple ID. The message on screen says which. Sign in the way you created the account, or reset the password: a reset confirms the email, and after it Google sign-in works for that account too. Registering from the link or code in your landlord's invite email confirms it as well. An account created with Google has no password to reset, so use the Google account it was created with.
After too many wrong passwords, sign-in pauses for up to an hour. Wait and try again, or reset your password so you are sure of it before you retry.