What does the なり do?
'高い/高いです' is as 'expensive'.
'なる/なります' is as 'become'.
'高くなる/高くなります' is as 'become expensive'.
Why 高くなります and not 高いなります？
That's simply how an い-adjective is declined (or whatever the word is) before the verb なる (e.g. みじかくなりました).
な-adjectives add に before なる (e.g. げんきになりました).
The word is conjugated - NP.
Also, I believe 高く would/could be written using kanji, but なります would be written using hiragana in the phrase 高くなります.
飛行機の切符がだんだん高くなります。why is it not accepted?
チケット (chiketto; ticket)
飛行機のチケット (plane tickets; plane's ticket; ticket of plane)
段々高くなります(becomes for expensive; becoming more expensive; is becoming more expensive; getting more expensive)
My answer came out as "飛行機のチケットはだんだんたくなります" but wasn't accepted. Did I make a mistake I can't see?
You are missing a か :) It should be ...たかくなります
Why is it not 高い (に) なります？
Hahah if they knew how cheap plane tickets would become due to covid19