The card needs too much power and its too tall so while it might fit in the slot the computer cover wont close because the card is too tall.  
  From the link below, the board is only 267mm in length.    [URL] One concern is that RX6800 works on PCIe 4.0 x16 slot.   R7's GPU slots are physically PCIe x16 but electronically x8. Not sure if that will work or not..
Aurora R7, can Radeon RX6800 XT fit in ?