When uploading CSV files there are several things that can go wrong.
(If you are looking for specifics on how to make CSVs for each category they can be found here:
Product: [Adding Products]
Stock: [Stock Consignments]
Order: [Adding Orders])
First make sure you are uploading a ".CSV" file and not something else.
Make sure you have the columns laid out in the correct order.
Make sure you have filled in all mandatory columns.
To check if you have your columns in the correct order consult the relevant "CSV Example" for CSV you are trying to make.
To check which columns are mandatory check the relevant "CSV Instructions"
These can be found on your portal, simply click the icon on the right to download each document.
A common thing to be aware of is Microsoft Excel, has a tendency to be auto format cells and this can cause numbers entered into them to be changed in unhelpful ways. For example, it will remove any leading 0s from the number, so 009257712 will be changed to 9257712. Also for particularly long numbers such as EANs it may and try and put it into "scientific form" e.g. 1234567890123 becomes 1.23457E+12. As this is now the wrong length the system will reject it when uploaded.
Dates are also automatically formatted to DD/MM/YYYY rather than the required YYYY-MM-DD.
To avoid this set your spreadsheet column to "Text".