Can a Canadian or US Citizen get entry visa at the check post between Hong Kong and Mainland China

Do you mean to get an on-arrival visa to mainland China? If not under an emergency listed on this page https://www.travelchinaguide.com/embassy/visa/service.htm, it's impossible. There is the possibility to get an visa there if he/she will just visti Shenzhen Special Economic Zone for less than 5 days and if he/she is holding Canadian passport.