Check the  below especially the highlighted part which shows the syntax for set ApprovarID:-

public class testApproval {

{

// Insert an account

Account a = new Account(Name='Test',annualRevenue=100.0);
insert a;

// Create an approval request for the account

Approval.ProcessSubmitRequest req1 = new Approval.ProcessSubmitRequest();
req1.setComments('Submitting request for approval.');
req1.setObjectId(a.id);

// Submit the approval request for the account

Approval.ProcessResult result = Approval.process(req1);

// Verify the result

System.assert(result.isSuccess());
System.assertEquals('Pending', result.getInstanceStatus(), 'Instance
Status'+result.getInstanceStatus());

// Approve the submitted request
// First, get the ID of the newly created item

List<Id> newWorkItemIds = result.getNewWorkitemIds();

// Instantiate the new ProcessWorkitemRequest object and populate it

Approval.ProcessWorkitemRequest req2 = new Approval.ProcessWorkitemRequest();
req2.setComments('Approving request.');
req2.setAction('Approve');
req2.setNextApproverIds(new Id[] {UserInfo.getUserId()});

// Use the ID from the newly created item to specify the item to be worked

req2.setWorkitemId(newWorkItemIds.get(0));

// Submit the request for approval

Approval.ProcessResult result2 = Approval.process(req2);

// Verify the results

System.assert(result2.isSuccess(), 'Result Status:'+result2.isSuccess());
System.assertEquals('Approved', result2.getInstanceStatus(), 'Instance
Status'+result2.getInstanceStatus());
}
}
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