Have tried about 10 different themes, and they simply will not install.

One simple sample here:
https://snipboard.io/kjEbzn.jpg

Operating System: KDE neon 5.23
KDE Plasma Version: 5.23.5
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.3
Kernel Version: 5.11.0-46-generic (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Core™ i5-3210M CPU @ 2.50GHz
Memory: 7.7 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4000

It looks like one of the component parts of the Global theme is missing on the store, or inaccessible.
if you go to the url of the item in the error- kde-look.org/<some numbers> you might be able to see if it is missing.

Perhaps the global themes need to be updated by their creators, or the website is being flaky.

Not sure.

Tried to install Ant-Dark KDE (worked with FerenOS) and it gave same error message but with different numbers at the end.

Frameworkintegration incompatibility? You could try installing it ( them) manually. Extract the downloaded zip or tar to ~/.local/share/plasma/look-and-feel/.

@dzon - tried to unzip one (took me a while to figure out how to su the folder).

Not sure it works well though.
Depends on the theme i guess.

No it doesn't. Btw, what do you mean su? Just unzip/untar it.
Download the theme, unzip and place it in that directory.
Btw, ant dark is actually a kvantum theme. It'll work better if you install the kvantum theme, aurorae theme, plasma theme and color scheme.

Yeah, as mentioned on the theme page, better use the kvantum. Like so: https://imgur.com/a/XUslt0G