«Якщо» introduces a condition: «Я не знаю, якщо це правда» = 'If it's true, then I don't know [something]. If it's not true, then I might know [something]. The sentence doesn't mention what exacly I might or might not know.' The sentence introduces a condition under which 'I don’t know' is true.
«Чи» is a marker of the question, especially when it's a subordinate clause embedded inside other question. «Я не знаю, чи це правда» = 'I don't know about the trustworthiness of this'. «Чи» introduces a sentence that is an object of 'I don't know'. There are no conditions: 'I don't know' in any case.