If I wish to build a playlist by searching for a track i can locate the track but sometimes the done button is not displayed so I can’t add it? Why?
You probably mean right after searching for a track, right? In this case, please hit “Cancel” first, then you should be able to see “Done”.
By the way, in djay 2, you can also add songs to the Queue directly from the main library. Simply tap one of the load buttons to open the library and swipe your finger on a song from left to right to add it to the Queue.