'limited knowledge' is like 'little time'
'a limited knowledge' is like 'a little time'
they are different in meaning/nuance; +/-
We often say 'speak/be knowledgeable on a topic/subject' but not 'have knowledge on a topic/subject'.. it seems weird, somehow. 'of' seems best is this case, but
'I don't know much about that' is more common.