Honestly, it could be a lot of things. The stuff you mention is a good first start. Make sure the posting period is not locked and that there isn’t a date/period mismatch. By spelled correctly, I’m also assuming you mean it’s in the correct format e.g. “Mar 2020” (you have to create a custom format in excel to get this to save to csv properly, alternatively you can map to the period internal IDs, or set a default on the mapping just to be sure the formatting isn’t an issue).  I hesitate to lead you in this direction, but if you’re absolutely  sure there are no issues with your posting period, sometimes you get these reference key errors when something else is wrong with other values you’ve set on your import. Things to check in that vein are that the customer/vendor/employee is not inactive, that the customer/vendor/employee is valid for the specified subsidiary.  Keep poking around, but I can assure you there’s definitely something incorrect in your file. If all else fails, try entering a few that are returning errors manually (using your import file as a guide) and you should run into the issue and more easily be able to see what’s wrong in the UI.