When the Message tab is greyed out, it is not about the ticket, it is about whether the customer can be reached. The composer falls back to Note, so you can still record what is happening internally.
Reason 1: there is no customer to message
A manually created ticket has no linked chat and no customer attached, so there is no one to message. These tickets support internal notes only. To reply to a customer, work from a ticket that was converted from a chat.
Reason 2: the reply window has closed
Chat channels like WhatsApp, Instagram, and Facebook only let you message a customer back for 24 hours after their last message. If that window has passed and the customer has no email on file, the tab is disabled. You will see a banner such as "No reachable channel. Replies will be saved as internal notes."
What you can do
Add an internal Note to keep your team updated.
For a WhatsApp chat past the window, use the WhatsApp template option to reopen contact.
Collecting customer email up front (which tickets turn on for you) avoids this, since email has no 24-hour window.