hey guys/gals just wondered if you can influence best positon ,cops what im finding regularis I promote someone from youth set up,and fopr example they are QB so I start training them in passing and then a few turns later there best positon is dline even though passing traing has been progressing well.

can we set players best positions or is it purely just random and we gotta roll with it

IF you look at your player's page. There is a tab for positions. This tab page shows all the player's ratings at all positions. Best position on the players main page is just showing the player's highest position.

As suggest above, just play the player in the position you want, and the best position will change over time. A player normally gains on average of 0.5 star of position per 10 games played. Sometimes more, sometimes less.