an add-on from a repository
We're curious. Which addon/repo is this about?
For log files, start here: HOW TO:Provide Logfile - LibreELEC
I would have thought the error message is obvious - it's banned.
LE does not and will not support banned nor illegal addons.
My apologies, We learn from our mistakes 
if I am not mistaken Freplay is not banned (unlike the repo where I find it)
Where can I get and install Freplay or equivalent addon?
TIA
How are you trying to install FrePlay? It's possible it's the repo and not the add-on that is banned.
Which repo are you using?
I would have thought the error message is obvious - it's banned.LE does not and will not support banned nor illegal addons.
You're confusing the heck out of me there. 
There are banned lists - but they're things that are banned for discussion / support.
In other words they're banned on forums, not within kodi.
There's no specific error that will pop up if someone tries to install from a 'banned' repo.
Maybe I've misunderstood what you're saying?
@ the OP...
If you can install from some repo's but not from a specific one that means there's a problem with that repo.
If it's a banned one you'll have to look elsewhere for support.
If the addon itself isn't banned (and a quick google suggests it's fine to me) you can always install from a standalone zip file - on the kodi forums that's seen as okay, so I imagine it's the same here.
Did you enable "Unknown sources" in System settings->Add-ons->Unknown sources?
And be careful of what you install!
EDIT
I just saw the addon you're trying to install on Github you might want to get it from there before installing a repository that is not so trustworthy.
