If you are unable to receipt a Purchase Order because the Apply button on the Receipt screen is greyed out it is because you do not have permission to receipt the order.
This is dictated by either your Authorisation limit or if you are able to Authorise orders of this Order Defintion.
Fortunately, both are controlled in the same area and can be changed easily with the relevant permissions.
Click System Admin, User Setup
Find and select the user you want to be able to approve the order
Select Company
Select the Properties option
On the POP tab, click the Authorisation tab and enter the Order Definition in the Definition column with a Maximum Order Value of at least the value of the order you cannot receipt. This will allow you receipt the Order.