I'm trying to set the ability to allow domain logins to my vCenter server.
I've done it on two other vCenters but I can't see the option in this version...
Is it a license restriction?
No, there's no license restriction. Ensure you're logging into vCenter with SSO domain administrator credentials. It should be in the SSO configuration portion. If you're not seeing that, show some screenshots of your environment.
I did have to reset the firstname.lastname@example.org password using the following post from VMware as someone else set it up.
I haven't restarted the services after password change - would that help?