The correct sentence would be: "When I was in Canada, I sang in the choir." Since both events (being in Canada and singing) happened in the past, you would use the simple past.

It should read. . . When in Canada, I sang in the choir.

Hi ホタルさん. Ok I will memorize the sentence that you corrected! Thank you.

I'd say "When I was in Canada, I sang in the choir."