Why can't I submit more than 1 file for an assignment/assessment in Moodle?
When a teacher sets up an assignment/assessment to be submitted in Moodle, one of the setting options allows the teacher to choose the maximum number of uploaded files that can be submitted for the assignment. If the teacher leaves the default setting of 1 this will mean that only 1 file can be uploaded for the assignment/assessment in total.
A teacher can send an email to the @ILLAHELP helpdesk to request changing the setting for the assignment/assessment (if there aren't yet any submissions).
Any student who might need to upload more than 1 file and cannot do so, should contact their teacher first a foremost.
You can also have a look at these Instructions for file submissions in Moodle.