Symptoms
You receive the error below when you attempt to use the chat feature as a guest in a Teams meeting. The chat icon is also grayed-out.
Chat in channel meetings is only available to team members.
Cause
The meeting is associated with a channel.
When a meeting is associated with a channel, this feature is not accessible for guests. That is the way Team's is designed to ensure only members of a channel will have access to channel related items. It also protects channel meeting notes and channel files from guests.
Resolution
If the guests need access to the chat feature, a new meeting will need to be created that is not associated with a channel.