There are three different Exchange Online Plans:
Exchange Online Plan 1
Exchange Online Plan 2
The Kiosk Plan which is included in K1 for Enterprise does not allow rules. You will need to upgrade or add a another Plan containing Exchange Online Plan 1 or 2.
You could just buy one Exchange Online Plan 1 and assign that to the same user as one user can have several Plans assigned.
Would take away the Exchange Online-Kiosk Plan from your K1, but otherweis your good to go.
This is to maintain the SharePoint Online-Kiosk from K1 as the Exchange Online Plans doesnt have this.
But depending on your prices it might be better to go for a E1 Plan.