Cannot connect to the App Store on my Mac running Mojave. Anyone else have this problem or a solution?

I'm having the problem, as well, on two different systems and a (local) friend of mine is too. It seems to be a regional outage, since I'm in the Eastern USA and my co-workers in Seattle, WA with Mojave are not experiencing the issue. The Console logs make it look like a "certificate pinning" error, so I'm guessing that there's a propagation issue with a new or expired certificate. I have a local friend with Catalina who is not experiencing the issue.

I called Apple and they basically refused to help me, despite AppleCare+ and a very expensive computer used in a production environment (where updates need to be carefully considered), unless I update to Catalina. That appears to fix the problem, if you want to go that route.

If anyone at Apple is listening, here's what I see in the Console from the App Store app (AppleMediaServices framework):

AMSURLSecurityPolicy: [9E11A8C3] Trust failed extended validation (EV)

This later causes:

Task <BD37D854-EED8-4C6F-AD00-0DDFA73657E9>.<1> load failed with error Error Domain=NSURLErrorDomain Code=-999 "cancelled" UserInfo={NSErrorFailingURLStringKey=<private>, NSErrorFailingURLKey=<private>, _NSURLErrorRelatedURLSessionTaskErrorKey=<private>, _NSURLErrorFailingURLSessionTaskErrorKey=<private>, NSLocalizedDescription=cancelled} [-999]

Followed by:

AMSURLSession: [9E11A8C3] Task completed with error = Error Domain=NSURLErrorDomain Code=-999 "cancelled" UserInfo={NSErrorFailingURLStringKey=https://bag.itunes.apple.com/bag.xml?os=OS%20X&osVersion=10.14.6&deviceClass=Macintosh&product=MacAppStore&productVersion=3.0&profile=MacAppStore&profileVersion=1&storefront=143441-1,13&format=json, NSErrorFailingURLKey=https://bag.itunes.apple.com/bag.xml?os=OS%20X&osVersion=10.14.6&deviceClass=Macintosh&product=MacAppStore&productVersion=3.0&profile=MacAppStore&profileVersion=1&storefront=143441-1,13&format=json, _NSURLErrorRelatedURLSessionTaskErrorKey=(

"LocalDataTask <BD37D854-EED8-4C6F-AD00-0DDFA73657E9>.<1>"

), _NSURLErrorFailingURLSessionTaskErrorKey=LocalDataTask <BD37D854-EED8-4C6F-AD00-0DDFA73657E9>.<1>, NSLocalizedDescription=cancelled}

My read on this, as an iOS developer, is that there's a certificate that didn't match what you expected, so you failed the authentication challenge and cancelled (-999) the connection. That mimics what I do in an app in which I use certificate pinning.

im having the same issue with my computer and my iphone 6 andi called apple and they couldnt really help me and i have no idea whats going on. its honestly really stressing me out. i cant sign in to my apple id either. it accepts my apple id but wont sign me in.

its actually all completely fixed now it must have all been that regional issue thing

The problem now appears to be fixed.

Yours sounds like a different problem, but you might want to wait a day to see if it magically resolves itself.