(also posted on Apple community support forum)
I bought a second-hand Mac Pro 5,1 (ex- 4,1) which had Mojave installed . I could not get App Store to download any apps - various error messages including "Download cancelled". I am able to successfully sign on to my apple account through the web and App Store "view information" prompts me for my AppleID and I successfully log on. So I have an active AppleID connected to my Paypal account.
I have successfuly installed freeware apps to the MacPro/Mojave, such as Unigine Heaven, outside of the App Store.
I reinstalled Mojave and have booted from the new drive - having removed the previous drive - so I know the Machine is running from a "clean install"
For what it is worth , the machine has 24Gb of RAM and a Sapphire RX580 video card.
I have copied part of the console log - which contains messages relating the App Store - but I don't know if these indicate any errors.
There are many postings on various forums about problems with accessing Apps Store over Mojave and I have tried all the suggested fixes which are "guaranteed" to fix the problem - and none had any effect. For example, the suggestions here: Mac App Store: Downloads just don't start (Mojave)
Do any of the attached messages give a clue as to what the problem might be failing to access App Store?