Hi @rparungao3
Currently, there is no function in flow to @all members in teams channel just using the channel email.
And if you want to use <at>email</at>, make sure you put only one person’s email for one <at></at>. You will need to add several <at></at> to @ several people.
You can go to ‘Power Automate Ideas’ forum to realize the function in the future if the post gets more kudos: Microsoft Teams notification when post on channel - Power Platform Community
But I can create a flow meeting your needs, though the flow needs more steps, as below:

First, if you want to @all the members in a channel email, you can get all the group members’ emails by ‘List group members’ action.
Then, initialize variable. And append to the string variable to add elements on the original basis.

Last, the content of message you want to post as the flow bot to a channel is the string variable.

Best Regards,
Community Support Team _ Kira Xie
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.