and when I am trying to print, received an error unit invoice form is invalid, missing or in use 
printing will not continue 

Your Invoice will only show up in Aging after it is posted. For Quick Bills, you can only Post after you print. If you are having an error printing due to unit format, you will need to adjust that before moving forward. Hope this is helpful. =)

thank you for your reply, how can I adjust the unit format?

Do you need it to be an Unit-based Invoice, or can you change it to Non-Unit Based Format?

..also, it might be a problem with the Report, so check Report Designer.

Check out the Sage Help in the program and search for Quick Bill Invoice Forms.

Thank you for your help, I tried both form Unit-based Invoice and Non-Unit Based Format and still have the same issue 

It is probably a report issue, then. It's why it can't/won't print.