How do you set ms playlist on manual mode
 

when i had ms. 4.20 and when i'm trying to learn a song i used to stop the song before it ended and also the songs on the playlist gets deleted after it ended if played to the last sec.but since i upgraded to 4.21 i noticed that it goes on auto play, tried to look on how to set it up on manual but can't find a button for it,

MS has never deleted the songs from the playlist that I can remember. And there is no button to change it to manual. It has run automatically (once you click play for the first one) for several versions now.
Try putting the same song in the playlists multiple times then you can practice over and over. Or stop it before the end so you can restart it as you say you have done in the past.

it must be me then, caused i could swear that on earlier version if songs are played to the end it gets deleted, that's why i had to stopped it before it ends, that's probably the reason why i didnt noticed that ms is an auto. mode once you clicked play the first time , and i know that there was a time that you could add /drag a song while a song is playing , coldnt do it anymore..but anyway if i'm wrong i apologize:)