new MacBook, can't sign in to AppStore, can't update to Mojave 10.14.5

Have a whole bunch of problems. New MacBook Pro (delivered last week) - bought a software bundle through education store with it, got all set to redeem the codes in the App Store. The App Store will not allow me to sign in. Any time I try (from the menu or from the bottom left), nothing happens. I can sign in to iTunes and iCloud just fine, so it's not the ID. Figured I should update Mojave in case that was the problem - got about halfway through the download, left home. Came back and it had the message that it couldn't complete the update. Thought it might be slow internet, took it to a place with faster connection. No dice - let's me choose to update, I ok to restart, it goes straight to "cannot complete" message and won't show me any details. At a loss. So far I have:

• Run first aid
• made sure there isn't a duplicate App Store
• signed out of iTunes and back in, out of iCloud and back in. No change.
• something with a digital certificate (Never trust, restart, System default)
• deleting some sort of library file that I found in instructions
• restarted about a million times

Please help! I need this up and running for a music education program asap! Frustrated because it's brand new, there's literally nothing on it besides the native apps.