Hello Julie
Can you go to the cog in the top right of QuickBooks Online,go to account and settings,expenses and then go to purchases orders and click into that section. Can you check if you have custom transaction numbers turned on? If you do not if you can turn it on and then save that section.If you then go back to the purchase order it should have a purchase order number box in the top right to enter the number you want.
If you do have that section turned n let us know
Thanks
Emma