First of all, this has nothing to do with whatever files you've received from the other Architect especially after you've uninstalled & reinstalled with just opening AutoCAD itself the popup occurs. Autodesk is basically telling you that your software is not legit.
Also it has nothing to do with the drawing version being 2018 or 2013. Since you're running AutoCAD 2018, it should be able to read all dwg formats without any issues since as I stated in my previous posts 2018 is the latest dwg format.
Looks like it's pretty obvious that since you bought this from an on-line store 3 years ago which would have been 2020 and since Autodesk no longer sold AutoCAD 2018 since 2017, then the party you purchased 2018 from gave you pirated software. Unfortunately, I have to say you've been duped. You can read up more about pirated software officially on the Autodesk site:
https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/The-license-you-re...
If you want to use real AutoCAD software unfortunately the only way to go now is to purchase directly from a certified Autodesk reseller or go directly to the Autodesk website.
And you'll have to purchase a new computer that can handle it.