Where do I enter non-resident tax withheld that shows up in Box 17 of my Canadian NR4 which reports my Canadian pension? I do not see an option to handle that.

If I understand this correctly, you are living in the United States and receiving a Canadian pension.

Per IRS Publication 915:

Under income tax treaties with Canada CCP, social security benefits OAS paid by those countries to U.S. residents are treated for U.S. income tax purposes as if they were paid under the social security legislation of the United States.

In other words, if you're a U.S. resident, you'd enter your Canadian CPP and OAS benefits as though you had received a Form SSA-1099 from the Social Security Administration.

Please note that the agreement only addresses government-issued social security benefits, not money received from private Canadian pensions