Why can't I put Google Maps on my dock?

How are you trying to do this?

I use WaterFox (Firefox). I highlighted the Google Maps URL in my browser bar and dragged it to my desktop. It made a .webloc file which I then dragged to my Dock and it added it. If I click on it, it will open Google Maps in my browser. It did actually make it to be opened by Safari but I used Finder's Get Info to change the application to open that particular file.

I'm only using Mac OS X 10.13.6 High Sierra, but I don't think Dock behavior has changed all that much in Mojave. Having said that, let's clarify that there's a small divider (a vertical line) in the Dock that designates the Application side and the "Other" side (where you can place folders, weblocs, and other, non-App items.)

So I'm assuming you knew this, and were trying to place your Google Maps item to the right of the Divider, correct?

If not, try again, and:

1) Click once in the URL field at the top of your Browser

2) Click again on the icon (far left of the URL field)

3) Drag that icon to the RIGHT of the divider in the Dock.

4) Give the other items in the Dock a second or two to "step aside" making room for your new addition

5) Release (mouse or trackpad) to drop your Google Maps icon into place

I hope this helped