In Messages the iMessages account needs an Apple ID
This is an emails that works with iCloud.
It does not have to be the same Apple ID as you use in iTunes or the iCloud setting in System Preferences > iCloud.
It will will be an Active Email address on the whole.
If your Apple ID is an old emails that does not work but is valid as an Apple ID then it is possible to have an Email that is not active but still use it as an Apple ID.
When you first add the Apple ID to the iMessages account then it can only send as the email address/Apple ID
If you have an iPhone then it is possible to send as if it were the iPhone in the setting for "Start New Conversations from" drop down.
It can be that the app does not login particularly after an update or upgrade to the OS.
I mention all of this as I am not entirely sure what you mean.
8:39 pm Monday; March 12, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)