You can complete purchases in two ways, through the Shop or through Regular Pass or Premium Pass.
If you become unable to use your premium pass while it is still valid, the Complete Purchase process may resolve the issue.
For detailed instructions, please see below.
Completing Purchases through the Shop
Home screen (leftmost button at the bottom of the screen) > Shop > Purchase Poké Gold > ( i ) button at the top-right of the screen > Complete Purchase
Completing Purchases through Regular Pass / Premium Pass
Home screen (leftmost button at the bottom of the screen) > Regular Pass or Premium Pass icon (third button from the right at the top of the screen) > Complete Purchase