This could be a few things. In general, this is an error that Shopify throws when they aren't able to accept the data that we are trying to import. From there, though, it will take a little sleuthing to figure out the root cause. One of the most common reasons that we've seen is if you include one piece of data that relies on another piece of data, without also supplying that second piece of data. For example: including a shipping method, but forgetting to set a cost for the shipping.
In general, this one can be solved by just reviewing all of your mapped columns and making sure that each piece of required information is there, and any that logically depend on another piece of information have that required information supplied.