Can You Say 'a Knowledge'?

We don't say "a knowledge". However, "knowledge" is sometimes used with "a", but only in the pattern "a knowledge of something".

'Cut in Line' or 'Cut in the Line'

It's "cut in line". We use the expression "cut in line" when someone unfairly goes in front of other people who are waiting in line.

'Wait in Line' or 'Wait in the Line'

We say "wait in line". When talking about a row of people or things, the article is usually, but not always, unnecessary.

Can You Say 'a Content'?

"Content" is typically an uncountable noun, so it's incorrect to say "a content".