macOS is "Unable to Check for Updates"
Seems like the App Store may be having some problems today.

https://twitter.com/macrumors/status/1091369924847558658

https://www.apple.com/support/systemstatus/

I experienced the same problem on a Mac mini (2018) running macOS 10.14.2.

Based on the comment from JBis, using softwareupdate --list was useful to confirm that an issue exists.  

$ softwareupdate --list
Software Update Tool

Finding available software
The operation couldn’t be completed. (NSURLErrorDomain error -1012.)


On a MacBook Pro running macOS 10.12.6 (Sierra) I was getting similar behavior.  The difference on macOS Sierra was that the App Store Updates tab would generate a popup indicating that "An error has occurred", which also included the last line of text from the softwareupdate output.