Unable to send message from my MacBook and iPad to a group Message

I have a one groupchat that i can't send message by my MacBook and iPad. It gets synced if I send message by my phone but I cannot see the message box that I can type in macook and iPad.

iPad software and MacBook software is both up to date

before I noticed this has happened, there were 5~6 notifications on the groupchat saying that you left the conversation within 15mins but I am still in the groupchat so I don't know what is happening. Is there a way to fix this?

[Re-Titled by Moderator]