The scan took quite a while to complete, but eventually finished. see Attachment
I have been using Avast for about 15 years, first I have seen this.
It has actually been like that for a few program updates.
See this and the links give in it:
Thanks DavidR Much appreciated.
I did a quick search found nothing, I’ll have to start doing a more thorough search in the future.
You’re welcome, not the easiest thing searching the forums.
What may help you in the future, do a google search but limit it to the avast forums, you can use the site limiter ( site: domain name, the space after the site: is important) to restrict it.
Example your topic title “Avast Scan says in scan history did not complete site: forum.avast.com” but without the speech quotes which are just showing what you should enter in the google search.
This is the result of the above search - for Avast Scan says in scan history did not complete site: forum.avast.com
https://www.google.co.uk/search?q=Avast+Scan+says+in+scan+history+did+not+complete+site%3A+forum.avast.com
I personally would probably have modified that search string further to restrict hits - but as you see the results aren’t bad and a hit early in the results the second entry in the search is pretty good.
Thanks for the info. 
Defilingly will try that in the future.
No problem, hope it helps.