How can I unlock modules for individual students?
In general, you don't. However, there is a feature request currently on Canvas' Product Radar to allow this functionality: Unlocking Modules for individual students. You may want to head there and lend your support by voting or providing your use case.
There may be ways to work around it depending on what the prerequisite is. You didn't go into details, so I'll provide a few examples. You may be able to change a "score at least" to a "submit assignment" and then put a grade in the gradebook for them so that they can complete the previous module. You might be able to add a differentiated assignment for that student so that they can complete the previous model. You may be able to remove the requirement from the previous model that is keeping that student from progressing. If the rest of the class has completed the prerequisites but this one student hasn't, then you can remove the prerequisite completely.
If you drop the prerequisite or change the requirements, be sure to change it back after the student is in. If Canvas asks about relocking or allowing students to continue, be sure to choose continue. This is described in the Canvas Instructor's Guide: 4152,41433
Students seem to like the requirements as it gives a checkbox on the module page that they've done stuff, but it does provide some complications when situations like this arise. I've stopped using them because I want them to access things in a non-linear fashion and they need to be able to go on to the next unit even if they haven't done everything in the previous module. For other people, the modules with prerequisites and requirements works well.
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