Error: top level design entity " " is undefined

Hello,
"Error: Top-level design entity "*NAME*" is undefined"

This is what I did to correct.
1) check paths for spaces, none allowed.
2) check entity name can not start with number.
3) Assignment - Settings - General - Top-Level Entity - "..."
4) From bottom up. select next entity up. Apply - OK.
5) Processing - "Start Compilation"
6) When complete. Tools - "Netlist Viewers" - "RTL Viewer"
Don't know if this step is necessary. It's what I did.
7) Repeat from step 3) moving up for each entity in list.
when finished, one last time with first (bottom/original)
"Entity Name"
9) Done

This created what was missing that was needed.
I worked for me hope it works for you.