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 in your booking software? 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 be sold at the particular location you are setting the Landing Page up for?
Did you just create this payment option in your booking software? You may need more time to sync this data.