Hermes winged staff

That just means you can't buy a staff currently.
If you go to the horse you want to add the item to, choose the item you want to exchange the staff for, you will then get options on how to pay for the item. Select the staff icon and your staff will be used to equip your chosen horse with that item

Oh thank you so much

I just tried and it didn't let me

You can only use the staff as a payment for items which can be bought with passes and which can be traded in the exchanges.