To reprocude this, you need to create a scratch org with OrderManagement enabled https://resources.docs.salesforce.com/228/latest/en-us/sfdc/pdf/salesforce_order_management_implementation_guide.pdf
Relevant features in sfdx-project:
This should expose the relevant interfaces for ‘commercepayment’, which seems to be working, however they seem to be generated wrongly.
Here’s more information about the relevant classes/interfaces along with sample code to build a payment gateway adapter: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_commercepayments_sync_adapter_concept.htm
Note: Tried rebuilding OSTs (full and SObject) and invalidating caches but still receive these errors. The code deploys fine.