Add sender is blocked when every licensed seat is used. Extra senders keep a No seat badge and do not send until you delete a sender or buy a seat.
Read the badge and the banner
No seat is a license badge, not a sender status. The sender can still be Active or Draft. The badge title is "Not sending: no licensed seat."
Open Senders. Look for the No seat badge.
Open Settings → Billing. If you are full, it says "No seats left. Delete a sender or choose a plan with more seats."
If senders already exceed licenses, Billing names how many are not sending, for example "{N} sender(s) are not sending. Choose a plan to license seats."
To add another sender, buy a seat or delete one you do not need.
If Add sender is grey, it reads "No seats left". The gate message is "No sender seats left. Choose a plan in Billing."
Fix it
Owner or admin chooses a plan with more seats on Settings → Billing. You cannot license fewer seats than senders already in this workspace. After a trial ends unpaid, senders lose their license and pick up No seat until you subscribe again.
Account Protection blocks sends from a sender with no licensed seat. Unibox will not send from that row.