This is likely because you didn't give your rep access when creating that Sender.
Here's how to give reps access to a Sender:
Go to Settings (the rightmost page in the topbar, sometimes collapsed in the three lines icon).
Within Settings, switch to the Senders tab.
Hover your mouse over the Sender you'd like to give access to.
Click the "Edit" button.
In the pop-up that appears, select the "Users" field and add any Users to let them send from this Sender.
Hit "Confirm" at the bottom to save changes.
