For group direct messages in Slack, adding a ticket emoji triggers the following message (see attached image):

We weren't able to create your ticket. You can only use an emoji to create a ticket on messages added after the bot was invited to the channel. In private group messages or for older messages in private channels,
use a Slack Message Shortcut to make a ticket. More details here: https://plz.halp.com/article/qbj8mfxgew-create-tickets-with-an-emoji-in-a-private-channel

It would be an improvement to enable ticket creation using an emoji like in channels and (single user) DMs.

Workaround

On the group DM message, click ••• on the message and "Create a ticket" or find the Assist app on "More message shortcuts..." (see attached image)