Based on what you typed you may have missed the actual ‘Sharing’ part.
In the ‘Edit Folder’-panel of the folder you want to sync (this is on your MacOS, left-hand side in the GUI), go to the ‘Sharing’ tab and make sure the device you want to sync this folder with/to is selected (FairPhone).
If this wasn’t done yet prior, on your phone a notification will pop up that a device wants to share a folder with you. Click on it and make sure to go through the settings there and apply it to set up the actual syncing process (most important is selecting a folder which is to be used on your phone).
If you hover with your mouse over that label which I marked in red, you should see some explanatory text.
In my case it says “1The remote device has not accepted sharing this folder”.
This would indicate that your phone never accepted the request to sync that folder. Some notification should have appeared. If not, retry syncing the folder (on your MacBook) with your phone as explained earlier (but then basically unchecking → save and then rechecking the checkbox → save).