To overcome this you must be rigorous in your approach and check all the functions, links and attachments. Take time to familiarise yourself with the portal and test it with draft documents before the final submission date. No matter how bad the portal, it is your responsibility to access the relevant parts.
Some tender portals are reasonably effective but many are confusing and cumbersome to use. It is very easy to miss a document or overlook an instruction on screen and the results can be disastrous.