When setting up AssignIT to accept assignments your course coordinator has made certain decisions about the:
of assignments that can be submitted via AssignIT.
This information is shown on the Submit assignment screen as shown below.

You will be unable to submit your assignment via AssignIT if it is not between the minimum and maximum file size established by your course coordinator.
In such cases you can either:
You can only submit assignments if they are a file type that has been allowed by the course coordinator.
If your assignment is not one of the required file types you can either: