Hi @PhoebeMilliken , just to add to @Hildi_Pardo and @Chris_Hofer ;
In our institutions students can usually make multiple Canvas submissions but our regulations are that the latest submission is the submission that must be graded, and it is therefore essential that students upload ALL of their files in a single sitting.
They also cannot make a partial upload before the deadline and complete the submission in the late work period (the time between the Due Date and the Available Until Date). Any partial submission is treated as complete once the deadline passes.
We have found that neither teachers nor students fully appreciate this, so it is important that this is clearly spelled out in the assignment instructions. It's an inconvenience in the marking process to have to go hunting for files split across multiple submissions.... and more of an inconvenience when we have to ask our Academic Directors for permission to mark work spread across several submissions.
If your institution is new to Canvas, then the guidance and regulations may still be emerging based on how Canvas operates.