Basecamp with TOPO PRO map and fenix 6x pro solar not usable

First, your map isn't being loaded to BaseCamp, it's being indexed so BaseCamp can use it.  So when it starts leave it for a while, if you disconnect it before the indexing finishes it will start all over again.  Once it's properly indexed you'll be able to use it correctly.

Are you sure it won't work on an external USB? The map file must be in a folder named Garmin, did you skip that step?

Basecamp only gives an error if I try to use .img from external USB (map file in Garmin folder). It actually gives error when trying to use some open street map also. Why that might happen, should USB stick be empty and only have that Garmin folder? Basecamp shows in device section that there is the map but when trying to select it from dropdown menu, it gives the error.

But when I connected the watch, I left it for 2 hours to load/index and still map didn't show properly but you needed to wait forever if panning or zooming even a little bit.

Possibly a corrupt BaseCamp install, try uninstalling then reinstalling.  What version of BaseCamp and Windows are you using?

Basecamp 4.7.1 and Windows 10 with newest updates. So you think .img - files should work on USB stcick when located in Garmin folder?

Maps have in the past, it's always possible Garmin have somehow done something to prevent that, but I've seen no reports of that. I have numerous maps on a memory stick, both Garmin and third party maps.

I reinstalled basecamp and still the same error:

Stick is shown as a device and map  shown below it but when trying to select it from dropdown above the map area, this error comes.

I think basecamp does not index it at all.

I think I found an answer fro Talkytoaster as that specific map is from there:

The mapsets are generally useable directly in BaseCamp by simply hooking up your GPS to your PC/Mac and running BaseCamp (see caveat below). The map will install automatically.

This means that you don’t need to install the separate BaseCamp version anymore (unless you want to, or you have a Mac and a Fenix 5 Plus [all models], Fenix 6s, 6 Pro, 6X, ForeRunner 945, MARQ, or any other Garmin device that uses USB MTP mode instead of USB Mass Storage Mode).  Please Note: There are some features that only function in the full separate BaseCamp version.

However, if you have a Fenix 5 Plus (all models), MARQ, Forerunner 945, Fenix 6s, 6 Pro or 6X and use a Mac, then the maps will NOT work in BaseCamp when the device is attached. This is due to the fact that Garmin changed the USB mode to MTP (which is NOT natively supported on a Mac).

So question is, can that limitiation have a workaround?

Ah, that will be it then.  On many devices you can change them to mass storage mode but not I suspect on your watch.  However while that may stop BaseCamp reading from your watch it should still work from a USB stick, which is a mass storage device.

Incidentally I'm confused, you say you bought the Topo Pro from Garmin but your screen shot shows the TT map?  If that's the one you want just install it directly to your Mac as TT advises.

I'm having both talkytoaster and TOPO Pro and neither of those works with USB stick but works when wtach is connected via USB and problem there is TOPO Pro being so slow.

Maybe the real problem now is, why those maps are not working using USB stick.