I have Avast VPN on my PC. I also downloaded it (on the same account) to my phone. That done, my VPN on the PC suddenly was stopped; the Avast app says that my license no longer is valid, and that I should renew it. I then looked up my different licenses, and found the VPN license in the list, and it is valid. What has happened, and what should I do to get it functioning on both PC and phone?
It’s my understanding that The Avast Secureline license is only valid on one device at any time. 
Unless you purchased a multi device license.
https://www.avast.com/en-us/secureline-vpn#pc
That’s correct, and the OP hasn’t specified what license is currently active.