Unable to assign *********** as an alternative host because the user cannot be s

[Silvernightfall]

Can someone please help me? I am trying to make myself a co-host for a meeting and everytime I try to add my email I am getting the error message "Unable to assign "***********" as an alternative host because the user cannot be selected at this time" I don't know what to do.  I made sure co-host is enabled but I am continuously getting the error.

Thanks