There are usually two reasons why this could be happening.
1/ The app hasn’t been selected as the post-purchase app in checkout settings. You can read how to do so here
2/ The payment method used to check out is not a credit card
Shopify limits post-purchase offers to orders paid with credit card. From their website:
If you’re testing on a development/test store and want to avoid charging a live credit card, you can set your payments to Test mode and use a test credit card. Please don’t turn on test mode on your live store! It will stop customers from making a payment.
We recommend to check your checkout settings and test with a credit card as payment method to see your post-purchase offer appear.