If there is no submission for a student's assignment in the Speedgrader, it is usually indicated by a blank preview page with a message that says "This student does not have a submission for this assignment", screenshot:
If a student is reporting that they submitted an assignment but you are seeing the message above, then their submission was not successful.
As the instructor, you can check the student's Access Report. There you can find the assignment, how many times they viewed it, and how many times they "participated" with it (i.e. submitted to it). More information about the Access Report is in this Canvas Guide: How do I view the course access report for an individual user?
The student can also check for themselves to see if their submission went through successfully. They usually get a confirmation right after submitting (see: How do I know when my assignment has been submitted?), and they can also return to the Assignment submission to see the confirmation, date/time stamp for the submission, as well as a link to see the file that they successfully submitted. More information about student assignment submissions can be found in this Canvas Guide: How do I submit an online assignment? (see heading "View Submission Attempts")
However, if you see an error message for their submission, then please report the issue to bcourseshelp@berkeley.edu and include the name of the assignment and the student affected, as well as a link to the submission and a screenshot, if possible.