The files we ask you to upload are based on your answers to the previous questions, or in the case of manual balances are statements for the account in question, so in most cases should be relevant to your circumstances - if you don't have the document then it may be that you need to go back a couple of steps and revise your answers.
However, if you have definitely answered the questions correctly but you just don't have the necessary attachment, in most cases you will be able to hit "Skip" and then you'll be asked to explain why you didn't upload it.
If you aren't able to skip the upload, then it means the company that asked you to complete the report has decided that this is a critical document - in this case you should upload the most relevant documentation you have.