Hi Preeti,
You can check whether the Exchange rate is maintained or not in FI for OB08 for from CAD to GBP for direct Quot., If it's not maintain, system will throw the error as the Exchange rates are not mapped.
second and main cause also will be the Currency translation ratios is not mapped in OBBS. For Exchange rate type M, CAD to GBP 1:1 and vise versa.
later you have to assign GL account determination for Exchange rate difference Loss and gain account.
Regards,
Saurabh