Why can't I copy the downloaded map from opentopomap to Garmin folder?

Try another computer just for sure.

Normally this works perfectly.
Why it doesn't work for you, I don't know, but nothing can be seen on their little picture.

But is an .img file ?

Copying files to this folder is not prohibited, it is something with your computer, as others suggested, try different PC. 
But my question is, which Fenix model you have?

Why you just use Garmin Express and install this map from Garmin?

Because I can't drag the downloaded map to GARMIN folder, so I clicked mouse to copy and paste, then found the PASTE button show as grey, not allowed...

it's Fenix 7 Sapphire solar, bought in China...
I am also very confused that when I open MAP MANAGE-TopoACTIVE in the watch and try to add map, only Greenland, middle east&central Asia map is available.

In Garmin Express with computer, Topo Active shows lots of more other maps, but all are marked not allowed except the two mentioned above...you can see the check box is grey and I can not select.  ToT

I think you need contact your local Garmin support.
It looks to be related to your watch version.
I have Fenix 7X Sapphire and have no issue to load maps on it, doesn't matter manually or using Garmin Express.