Approval process won’t trigger record-triggered flow

Since the discountpercent__c is a roll-up summary. At creation your record won't have a value (you can't have children before you insert a record as you need it's Id).

What is happening is that you flow is calling the approval process, but the record does not pass the entry criteria and hence the error you found.

A way to avoid that is to make sure the entry criteria for your flow is the same as the Approval Process.

The reason you're seeing the message "You have uncommitted work pending. Please commit or rollback before calling out" is because Salesforce does not allow HTTP callouts during a record change process. If you were able to make callouts during record changes, database performance would be severely impacted, and record lock errors would become more frequent.

When it comes to external callouts as part of record-invoked automated processes, your best bet is to utilize a @future method. These methods will run asynchronously after the record update is complete. In your @future method, you can send the payload to your external service, process the result, and update the relevant records in Salesforce.

• As an aside, the scheduled action doesn't work for the same reason. Even though you're scheduling the process for some point in the future, you're not separating the database transaction and the HTTP callout, which is what we can do with a @future method.