I have a Yemeni passport and I have an electronic ETA for Canada.?
You should have your green card in your possession and show it to check in staff. Normally, Yemeni citizens must have a visa to enter Canada; US LPR status is what allows you to avoid this requirement and only need an eTA. When you check in online, use your green card and select 'US Permanent Resident as your citizenship. All airlines I have checked in with offer US LPR as a document type.