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

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

We need a knowledge of the facts as well as a proper assessment of the circumstances.

We need a knowledge as well as a proper assessment of the circumstances.

Follow the same convention when using an adjective to say, for example, "a deep/extensive/good knowledge of something".

Sarah has a deep knowledge of human nature.

While it's incorrect to say "a knowledge", you can say "a piece of knowledge".

That was a useful piece of knowledge.

That was a useful knowledge.

In most situations, nevertheless, "a" may be redundant when used with the uncountable noun "knowledge".

My father has extensive knowledge of biochemistry.

We often use a possessive pronoun ("my", "your", "his/her", "our", "their") or another determiner for uncountable nouns ("some", "much", etc.)

David kept his knowledge of the love affair a secret from his wife.

She already has some knowledge of thermodynamics.

Be also aware that the preposition that follows "knowledge" is "of", and less frequently "about", so we say "knowledge of/about". Don't use the prepositions "on" or "in".

He denied all knowledge of the events.

He denied all knowledge on the events.

Since it is an uncountable noun, you cannot use "knowledge" in the plural.

We need to strengthen our knowledge.

We need to strengthen our knowledges.