Can I claim a foreign tax credit in Canada for US tax withheld on income covered by the treaty?

Generally, yes, a Canadian resident can claim a foreign tax credit against Canadian tax for US tax properly withheld on income covered by the treaty, up to the amount of Canadian tax otherwise payable on that same income, which is exactly the mechanism that prevents you from being taxed in full by both countries on the same dollars. The credit reduces your Canadian tax bill by the US tax you've already paid, rather than requiring you to pay full tax twice and sort it out some other way.

The word "properly" matters here: the credit is generally available for US tax withheld at the rate the treaty actually allows, not for tax withheld in excess of that rate due to a paperwork error or a payer applying the wrong default rate. If too much US tax was withheld because the treaty-reduced rate wasn't properly claimed at the time of payment, the more appropriate fix is usually recovering the excess directly from the US, rather than trying to claim the full excess as a Canadian credit.

Keeping clear records of what was withheld, at what rate, and on what income makes claiming the credit correctly on your Canadian return considerably easier than reconstructing everything at filing time.