It's incorrect to say "knowledges". Since this noun is uncountable, it cannot be used in the plural.

It's incorrect to say "knowledges". Since this noun is uncountable, it cannot be used in the plural.

She knows how to use her knowledge.

She knows how to use her knowledges.

When "knowledge" is the subject of a sentence, use the verb in the singular.

Their knowledge of nuclear physics is extensive.

Their knowledge of nuclear physics are extensive.

We typically use the preposition "of" with "knowledge", and much less frequently "about". Don't use "on" or "in".

His knowledge of human nature is not innate.

His knowledge on human nature is not innate.

You can use the indefinite article "a" before "knowledge", but only in the pattern "a knowledge of something".

My sister has a broad knowledge of contemporary art.