Some channels need verification in order for data to be synced. Google and Facebook are channels that need verification. Simply click on the connect button, make sure to login with the account that is connected to the Google My Business Account and separately Facebook Business Page.
Click accept and you're done.
If the location is not yet been verified by Google before, it might be the case that Google will contact you to verify your location.
Have a look at the following articles to connect your Facebook and Google My Business account:
- How to connect your Google My Business account with Local Listing
- Connect your Facebook Business account with Local Listing