Why can't I change my password anymore? I'm trying to change my password for my Pressero admin user account by going to Preferences > Admin Users from the menu and editing my admin user, but I am not allowed to change my password?

If your Pressero admin account has permissions for managing users, you will notice that from menu command Preferences > Admin Users, you can reset the password on other admin accounts, but not reset the password on your own admin account. You need to make any changes for your own password from the Profile for your admin user.
On the blue bar at the top of Pressero admin, go to your account name on the right, click on it, and choose My Profile. There is an Update Password button available on your profile that can be used to change the password for your own Pressero account.