Centre Tyler Morgan, No 8 Tom Phillips and lock Michael Snook all ran over for scores with Luke Price adding a conversion.
But it wasn't enough in the end and Wales now play Portugal on Tuesday determined to bounce back strongly.
Lewis said: "The boys played very well in the first half, they were very direct, created chances and scored three good tries. At 17-3 up we were comfortable and continued to create opportunities but failed to capitalise on those and following an intercept ball, Scotland went 70 metres to score, taking the score to 17-10.
"Our players continued to play well and stuck to our ball in hand approach. But again, we didn't finish off try scoring opportunities and Scotland scored another good try to get within two points of us. They then converted a penalty in the final moments to win the game.
"Although we created plenty of opportunities, we lacked a little experience and composure at key moments and failed to take advantage of overlaps at times, our attack was sometimes too narrow.
"It is extremely disappointing to come away with a loss, a harsh lesson, but there are certainly positives to be taken from the match."