Can't approve travel expense in Fiori

Dear all,

I get the following error Decision 0001 cannot be perfomed on workitem 000006002944 with "Approve travel expense" application. I am using the workflow WS20000050. This problem has solve in this topic SAP Travel Expense Custom Workflow

But it is not clearly for me. So could you help me to clear this issue.

Another question, I know we can use 2 workflows WS20000040 and WS20000050. But how I can define SAP use workflow 40 instead of 50? SAP said that we can use WF 50 for one step approval and WF40 for two step approval (first step will be in done at Expense dept and the second step was approved by Supervisor).

Thanks so much, Regards

Vu Pham