Cannot Modify Active/Once Active Approval Process Definition in Salesforce

Here is way to delete an approval process if you received a below error "Cannot Modify Active/Once Active Approval Process Definition".

       1. Active approval processes can't be deleted.
       2. Before deleting an approval process, make sure it is inactive and that no records have been submitted for approval.
       3. If any records have been submitted, delete them and remove them from the Recycle Bin.

Refer the below useful links,

Considerations for Approval Processes

Unable to delete/add Approval Process Step