Quote Originally Posted by Insertusernamehere View Post
"Your retainer must have archived level 60 as a Disciple of War or Magic WITH a ARR JOB starts from level 1".
Are you sure it said job and not class?

I haven't tried switching a retainer to a ShB job yet but in Stormblood you could switch your retainer from a base class to either the upgraded job for their class OR any of the expansion jobs. Once you upgraded, you couldn't change them again.

Are you sure that you changed your retainer from Lv50 BRD to MCH, or was it from ARC (archer) class to MCH? You also mention having a "level 1 bard" which is impossible, so I think you're getting archer/bard confused.

Most likely, you need to level your retainer to Lv60 without upgrading from class to job, and upgrade to GNB at that point.