There are a number of reasons why this can happen.
First, please be sure that your file name does not contain spaces, punctuation, or special characters. BOLT doesn't like files with names containing those things.
For example, if your file is called Stark_assignment#2.docx, try changing the file name to Stark_assignment2.docx.
Sometimes you may encounter issues in the web browser you are using. You may want to try another browser to see if you can submit your assignment from a different browser.
Also, keep in mind there can be time restrictions on assignment submission folders. If your instructor requires the submission by 11:59PM, try to login to BOLT and submit earlier than that. If you login at 11:59PM, you may not have enough time to get to the folder and submit your file by the time the clock hits 12:00AM. Give yourself a time buffer to make sure you submit your assignment before the deadline.