I’d try contacting HR instead of your manager first. If they still refuse, I’d gather as much supporting evidence as possible, like employment contracts, pay slips, tax documents and any official job description. I’d also include an explanation letter stating why you couldn’t get the reference letter and attach proof of your attempts to obtain it. IRCC does allow explanations for unavailable documents, but acceptance depends on your application and the evidence provided. Check the official IRCC guidance for your specific PR pathway before submitting.