This error has come up here on the forum. Sometimes it's a permission issue, but check this thread and I would do as Sinbad had suggested and download the csv and try to upload without changes to see if it works. If it still does not, then I would check the server for a file size limitation.
Ok thanks Dave. Had seen that one and changed the values without success. However did continue to investigate the issue and in my case the correct fix was as follows
Product Inventory CSV Upload Resolution I went to IIS manager, selected website then under IIS selected ASP and then Limit properties and set 'maximum requesting Entity body limit 20000000 then clicked on Apply'
I now however have a new issue with productimages upload
Updating or Adding Row Microsoft OLE DB Provider for ODBC Drivers error '80040e14'
[Microsoft][ODBC SQL Server Driver][SQL Server]Incorrect syntax near '='.
I would test by downloading an image csv from your store and then re-upload that (without edits) to try to determine if it's something in your csv file that is causing the issue.