Cannot transfer ownership of Smartsheet Calendar

I am trying to transfer ownership of a Smartsheet Calendar to a coworker, but when I try, it is giving me an error message, saying that the user isn't found and new user needs to be shared to the source sheet/report as an Admin in Smartsheet.

I have checked, and this coworker is already an admin in all of the source sheets. I've also double checked, and I am spelling my coworker's email address correctly.

What can I do to fix this problem so I can transfer the ownership of this calendar to my coworker?