I'd suggest checking your web Settings page and ensuring that the Disable desktop screen sharing for meetings you host option is disabled. If this is enabled, this would explain why you cannot share your desktop. Keep in mind though, this only applies to meetings you host. If you still experience this in meetings you have joined, the host of those meetings would need to adjust this on their end.
Hope that helps and please make sure to mark the solution as accepted if this information is what you needed.
This happens to me, but only in one particular room with a particular HDMI cable. I checked my web settings as above, but that option was correctly disabled. Is there something about specific HDMI cables/connections that stops you from being able to share your screen?