The first time that you use the Screen Mirroring app on your Mac, your Samsung will ask you whether it should accept incoming connections. This looks like this:

 

Samsung TV Allow

 

Select “Allow” and the screen mirroring from your Mac to your Samsung TV will start.

If you have selected “Deny” by mistake, there is an easy way to fix it:

Samsung TVs from 2012 to 2014:

First go (on your Samsung TVs) to Network / Allshare Settings / Content Sharing, then select the device with “Denied” next to it. Click it and change it to allow. Then start the mirroring again from your Mac.

Samsung TVs from 2015 to 2017:

Go to (on your Samsung TV) to Network / Multimedia Device Manager, select the device with “Denied” and select allow. Then start mirroring again from your Mac.