Your Canada TRV is already in process. Do inform IRCC asap through the Case Specific Enquiry (CSE) without waiting to get a copy of the UK visa refusal letter.
For the CSE, you must mention:
1. You had a UK Visitor Visa refusal (state the date if you remember it, or else the approx. date when you applied)
2. You do not know the exact reason(s) for refusal as you do not have a copy of the UK visa refusal letter. I would not mention about a missing sponsor letter.... without seeing the actual UK visa refusal letter.... because if this is not the correct reason then it would complicate matters
3. You have not reapplied for any UK visa after that refusal
4. You are declaring this UK visa refusal on your own without IRCC asking you for this information
For the CSE, you must mention:
1. You had a UK Visitor Visa refusal (state the date if you remember it, or else the approx. date when you applied)
2. You do not know the exact reason(s) for refusal as you do not have a copy of the UK visa refusal letter. I would not mention about a missing sponsor letter.... without seeing the actual UK visa refusal letter.... because if this is not the correct reason then it would complicate matters
3. You have not reapplied for any UK visa after that refusal
4. You are declaring this UK visa refusal on your own without IRCC asking you for this information