richard.thomas,
To supplement what @stuart_ryan wrote, if you want to enforce the file upload before the student can proceed, then you can use the module system and make the file upload a requirement for completion of the module. Choose "student must submit" for the requirement.
Depending on your other needs, you can make students progress through the module requirements sequentially, so that they cannot do other things in the module requirements until they have completed the file upload or make completion of that module a prerequisite to starting another module.
Here are two lessons from the Canvas Instructor Guide that provide more detail and prerequisites and requirements.
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