Getting error you cannot associate the selected project with the selected task. The selected project

I tried to link the sub project ( project   to program ( program A), it gets the above error message.   Then,

for testing purpose, I deleted the top task from program A, and re-create a new top task, try to add project B to Subproject Association.  Same error message come up.  Then,

for testing purpose, I tried to link the sub project ( project   to program ( program , it works.  I delete the link in this step.

What I can research why program A get the error of 'You cannot associate the selected project with the selected task. The selected project already exists in the current project structure.'