I took over a shop that already had a value in the Google Analytics Tracking ID field. I don't have access to that account so I created a new one but it looks it's not as easy as just copying the new Measurement ID from Google Analytics over to the field in Shopify.
I was repeatedly getting an error message telling me my Google ID was not the right format and had to have an UA at the beginning (this is the OLD version of Universal Analytics which it seems that the NEW Google doesn't reference anywhere.)
So, I'm not sure if it's going to work but after some clicking here and there and reading a bunch of help file threads (the ones from Shopify are outdated and the ones from Google don't reference Shopify), I figured out that this is how you can get it to connect (although I don't know if it actually works, I'll let you know in 24 hours if I actually get any data to pass over).
1) Create a Google Analytics (GA) Account
2) (In GA) Create new property (shop name)
3) Create a new stream (I did a web so I could use my shop URL)
4) On the Web Stream Details page (in GA) scroll down to where it says Additional Settings>Connected Site Tags
5) On the Connected Site Tags page, copy your existing (OLD UA-XXXXX code), add a nickname if you want and choose connect This will then LINK your Shopify account with the UA code on it to the new Google Analytics dashboard (I think)
6) Go back to the Web Stream Details page and click add new on-page tag>Global Site Tag (gtag.js) and copy this code to your clipboard
7) Go to Shopify>Online Store Sales Channel>Preferences (I kept looking in the Settings but apparently that's not where this code lives)
9) SAVE YOUR CHANGES
After all of those steps, I finally stopped getting error messages on the Shopify page at least. I honestly don't have enough traffic to my site to know if this worked (until the day time at least).
If anyone wants to try these steps and let me know if there are any changes or updates needed, please feel free.