Assuming you are attempting to configure a Web Adaptor for an ArcGIS Server (you appear to already have a "portal" webadaptor configured)
Is "admin" the "Primary Site Administrator" account that was used when creating the ArcGIS Server site?
You will not be able to use a Portal user for this dialog. The WA is attempting to access the Server's token service and this will only work for a built-in account - most likely the Primary Site Administrator account (not the windows service account)
sorry about slow response - I was away from work for a few days.
Looks like Server is not even running - did you check the windows service?
Can you reach https://vie-lap-pantelis.esri.com:6443/manager ?
a typo - sorry - the first test link I suggested should have been
but if that doesn't work, please check that the Windows Service "ArcGIS Server" is actually running.
also, check from the Start menu - Internet Information Services (IIS) Manager
navigate down to Sites -> Default Web Site
there should at this point only be "portal" as a configured web adaptor
it is possible you have accidentally created an "arcgis" Web Adaptor that may be confusing your web requests