"To log in with this character you must first install Heavensward"

[Arazehl]

You need to register your second cd code for Endwalker. You can find the official registration through the Square Enix store via you purchases or your email. If you purchased through Steam you will find it in your games library by right clicking on FFxiv Online Game in the list and go to cd keys. There you will find the official EW key that you need to register.

If your no longer have the expansion you'll need to ask the support team to move your character to a safe area and you'll lose the levels you gained for early access unless you rebuy the expansion