Hey OP:
What I’ve found over the years is, you basically never hear a WoW forum poster come back and say ~“I may have been wrong here”.
So, let me say this… “I may have been wrong here”.
I made a Tauren Warlock the other day (because of course!), which brings my total characters up to I think 47 or 48. HIS Garrison Hearthstone (toy) works great.
Just to check and see… I then used my Garrison Hearthstone on every single one of my guys, and I found ONE guy having the same issue as you.
(it is my first and oldest Warlock, who would have been created in late 2005 to early 2006.)
Every OTHER guy, except that Warlock’s Garrison Hearthstone, works perfectly. So, on ONE of my guys it IS happening, and on the other (let’s say) 46 it is NOT happening, and they are all OK.
On THAT Warlock, he gets the same error message as you when trying to use his Garrison Hearthstone (toy)… “You can’t do that right now.”
So… is it your Add-on? I dunno.
Is it another/other Add-on? I dunno. I use a total of 6, and I’ve got 3 which were apparently not updated… Master Plan, Rematch, and Venture Plan.
MasterPlan → UPDATED → issue persists
Rematch → UPDATED → issue persists
VenturePlan → UPDATED → issue persists
I could completely remove Venture Plan, but I actually did do some Shadowlands stuff on a guy last week.
Is it the latest patch? Maybe? Probably? I dunno.
Is it some weird coincidental thing related to a Warlock? I dunno. I’ve got 4 x Warlocks, so I doubt it. It’s not spec, or Talents, or summoned pet status.
Is it some weird coincidental thing related to a specific character? I dunno. The possibilities there are nearly infinite.
If I have to guess it may be something related to Draenor progression in some way. No way (for me) to figure this out, though, as every one of my guys has “done Draenor”, some much more than others, and THIS Warlock would have completed a great deal of Draenor stuff, compared to many of my other guys.
But… it’s happening to ONE of my (47 total) guys too.
So, good luck, and I hope it gets corrected for you soon.
This is not a huge deal for me, but if instead of just ONE guy, it was happening on the other 46, I would be very very irritated, for sure.