Especially try to attach some file attachments - maximum single file size is limited, but you can attach more files one by one; to do so use the button to choose a file from your computer repeatedly.
By clicking the continue button you will be forwarded to the page for electronic signing of the tender - you will need an acceptable electronic signature (based on qualifying certificate) to do so. The system then verifies the validity of the signature.
More information can be found in the user's manual (pdf, 3,13 MiB), in the chapter related to electronic tenders.