The answer depends on whether your students are added manually or by your Student Information System. If it is the latter then you probably shouldn't (and your Canvas should be configured to prevent teachers from dosing so).
You need to get to the bottom of what the issue is - have you talked to your Canvas team/IT support?
If the student has not accepted the invitation they will normally show as 'pending' and you have the option to resend invitations.
If they still can't get in (or are not showing as pending) then there may be an anomaly around their ID or login that you need to get to the bottom of....