You can check your Google Play Store order number through the payment receipt sent to your Google account's email inbox or directly through the Google Payment Center.

[How to check the order number in the Google Payment Center]
Access the Google Payment Center (payments.google.com).
Log in with your Google account.
Go to Subscriptions and Services > View Purchase History > Select the order, then check the transaction ID on the receipt.
※ Please provide the order number starting with GPA or attach a screenshot.