As posted elsewhere, a GI-275 will not sync updates to a GTN - Only the other way around.
Now, as far as the initial question of the thread, here's what happened. There was apparently some sort of cybersecurity issue at Garmin that affected (or had potential to affect) the databases that are purchased from them. Garmin quickly and quietly started moving to an encrypted, signed database format. Of course, that wouldn't work without software updates on our individual units! Unfortunately, they didn't finish all of the updates at the same time, so if your avionics shop updated your software at the wrong time, you had some units that could use the signed format and others that couldn't.
I was tripped up by this during an update last year. I know that as of Oshkosh last year they still hadn't gotten everything working on the signed databases, but it sounded like they were close. I finally got around to getting all of my Garmin units updated recently, and now the database sync is working again.
So, if your database sync isn't working - Go to your Garmin dealer and have them update your software. FWIW, when mine was broken I was on GTN (non-Xi) version 6.72 and GI275 version 2.60. I actually didn't look at what the new versions are, I'll have to check next time I'm at the hangar.