I cannot transfer a map onto my Garmin

Does the *.IMG file appear on the watch? You can look at the time the file was created on the watch itself in the GARMIN folder. If the file appears, you need to look in the watch settings to see if the added card appears in the list and whether it is activated.

No the map file doesn’t appear on the watch. The mapinstaller goes through the steps of transferring the file from the Mac to the watch and says installation complete but when I check the watch there is nothing there

Which "Garmin" are you using?

Fenix 6s pro

I know it's not a real answer to your question but: have you tried using a PC? So that you rule out problems with the watch.

Try a Windows PC. If this is not at hand, you can run Windows under MacOS using Parallels Desktop for mac. 
Of course, there are other options, but Parallels was personally checked by me, the clock connection works 
correctly on it.

All you need is the MAC app "Android file transfer" and an openstreetmap map in ".img" format. You need Android file transfer as a Finder replacement, because mac does not recognize the watch.

Download a map in ".img" format and copy it into the GARMIN directory of the watch.

Good maps directly in img format can be found for example here :

https://www.userbeam.de

Thank you

Bluefish is correct, with some nuances. The Fenix uses the MTP protocol to connect the watch to the computer, that protocol was created for media files (music essentially) and Garmin adopted it when music started being supported on the watches because music apps use that protocol. Before it was using ubiquitous FAT format directly in Finder. So with the fenix on your mac no drive icon appears. The Mac supports MTP but not in the Finder. The Apple music app does. Also the Garmin ConnectExpress, BaseCamp and MapInstall   apps will also support MTP. Since MTP is limited to one connection at a time it gets annoying and you always have to make sure than only one MFP app at a time is running else one will get the connection and block the others. So many people want to do things the old way. To do that they use AndroidFileTransfer, which give you an interface to copy files much like you would do in Finder. So the two processes are as follow.

1) Using the .gmap file from OSM, use MapManager to "install" that .gmap onto your Mac using the Command-I command. Then use MapInstall to transfer that previously "installed" map from the Mac to the watch. The MapInstall will create a .img file in the process and transfer that file.  So with that flow you get a .gmap from OSM and end-up with a .img on the Watch.

2) use the .img file from OSM ( you can download one usually as a PC file), then use AndroidFileTransfer to transfer manually that .img file to the watch in the /Garmin folder. 

Either workflow can work, each with it's particularities. For example you may have to manually enable the transferred map in the watch activity profiles you want to use it with. You will have to work your way through. MapInstall can do a few things in the background while AndroidFileTransfer does nothing else than copy the file. I have been successful both ways.

Thank you that’s a very detailed answer, however, not being all that IT savvy, I’ve given up and just bought the map from Garmin… it’s working fine. But thank you again for your efforts