I am using a Macbook Pro (2018, 13 inch, 2.3ghz i5) from work, and the previous user had installed Xcode and a few other apps through their apple account. I have since deleted Xcode, but whenever I try to download Xcode again through the app store, or any app for that matter, I get the message "We could not complete your purchase - Unknown Error". I have signed out and in, restarted several times, and looked in the Error Console. The console gives the error "Purchase failed with error: " and I can't find more. Any help would be greatly appreciated so that I can finally get back to work!
The reason for this is that:
Your Xcode application was installed previously with a different Apple ID. I would suggest you to check if there is any account active still in @Internet Accounts. Remove it and also go to Profiles and check if there is MDM profile installed or not. If you have an MDM profile, you have to reach out your IT to fix it.
Also, sign out from Mac Store and iTunes, reboot and try to sign in back if there is no MDM profile installed on your Macbook.