When you are not able to log in, the first thing you may need to check is if your username and password are correct. After checking your sign in credentials, if you are still unable to sign in, the suggestion would be to reset the password.
If you are unable to sign in to our iPhone or Android apps, the reason could be that you are accessing with your Google apps, Yahoo, Google or Facebook credentials. To sign in to the iPhone and Android apps, you will need to set up a Zoho password. To do this, you must access the website from a laptop or a desktop and follow the instructions suggested for retrieving a password.
Zoho Invoice FAQ