Hmm, strange, though not unprecedented (see: [1] [2] [3] [4]).
Could it be that the content that you are trying to stream is a copied or downloaded file, but not from Apple servers? Or could it be that you are trying to stream Apple content while offline? Could it be that the format of the video file is supported by the sending device, but not by the receiving device? (The Apple TV app on Mac is a bit more versatile than the AirPlay receiving devices.)
Could it be that you are trying to stream a Home Sharing file on the Mac, through iPhone, with AirPlay from there? Such a double hop wouldn’t work. Stream from the device where the file is located.
This issue 🚫 is also known to happen for some web videos, where the content provider didn’t make provisions for AirPlay compatibility in their streams, or blocked AirPlay use.
If none of the above applies to your situation, then I would suggest to contact an expert from the Apple Support staff for further assistance. Contact Apple for support and service (phone numbers) or start your support request online (chat/email/callback).