As you may have already know there are are two ways add custom domains in hashnode:
CNAME record
A record
So I also couldn't add the CNAME record. It gives the following error :

So I did it with the second method and it works. The only downside is with this method, we lose the benefits of global CDN and edge caching of hashnode. So anybody find out the way, please share here.
Okay so here's the instruction from hashnode for adding A record :

And you just need to add it in the google domains like this.

And the end result will look like this. And you are all set. Happy blogging :)