There could be a few reasons why the package or membership you are looking to sell is not pulling into your Landing Page drop-down:
Is the payment option active? While payment options do not have to be public in your booking software, they do need to be active.
If you have multiple locations, is this payment option set up in your booking software to sell at the specific location you are setting up the Landing Page for?
Did you just create this payment option in your booking software? You may need more time for this data to sync.