If you already had SQL Express installed, most likely it was installed with configuration only for Windows Authentication and the PI wants to use sql authentication. They have made it such that if you install SQLExpress using the PI it configures it for Mixed Mode authentication which allows both Windows and Sql auth, whereas the stand alone SQLExpress installer does default to Windows only.
You can manually configure your existing installation to support Mixed Mode but then you won't know what the sa password is (it might be blank). But once configured for mixed mode you could create a new sql user named admin and add it to sysadmin role under the Security node, create a known password for this user then use it instead of sa.
Then you should be able to get the PI to work. The PI will also enforce that the dbuser for the app have a strong enough password to match the default password requirements. It needs at least 1 special char 1 upper case 1 lower case and one number so something like $Secret123 should work.
Hope it helps,
Joe