Provider and receiver project association in the contract

My case that I have two different LEs and BUs which they are having intercompany transaction. Now, under the project LE 1 will be the provider and LE 2 will be the receiver. I created two projects one for each PU and one contract between the provider and the receiver then I assigned the receiver BU and the receiver project, however, I am not able to select any project when trying to associate the provider project to the contract line.