I've been having this issue too and has been so frustrating. HOWEVER i managed to get it working (no thanks to the lazy answers to make sure you are using the templates and complying with the "Completing the product import .csv file" support page)
With Excel, there are multiple CSV Save-As options. If you use the "CSV UTF-8 (comma delimited)" option, it wasn't working. So instead, i used the simpler "CSV (comma delimited)" option and it worked first time. Would like to see this documented somewhere on the Squarespace support.