Some hosting providers have a different process for users to log into their WordPress account to manage their sites. Usually, you would click in a Log In button directly from your cPanel that would automatically connect you with your WordPress site dashboard.
If this is your case, you would need to follow the steps below to log into the WordPress Mobile app:
First you will need to create a new administrator user (under Users > Add New) on your site. Then, sign into the app as that new user.
To see how to create a new account for your site please check out this link here
Once you have your new account, to access a site that is self-hosted, from the Log in section, when you first open the app, click on the link that says “Enter your existing site address.” From that option enter your site address, and then your username and password for your site.