Can not create Invoice Proposal for a Service Order

Hi All,

I try a basic testing as belows:

• Create Service Order, select an existing Project - Time & Material
• Create Service Order line - type Expense  & enter Sales Price
• Post Service Order  -> Done
• Create new Invoice Proposal for this Service Order:
  There is no line for me to select
  

Can you please let me know why there is nothing for me to create Invoice Proposal while I already done to post Service Order? 

Thank you in advance.