Why do students receive an "invalid" message when trying to log in and vote?

In order for students to vote, they must be on the eligible participants list for the poll. Otherwise, they will not be able to access the poll from the Student App or via voting link authentication.

To add students to the participants list, click the Globe icon next to the voting poll name and use the "Add from List" or "Add from IDs" option to select students. Student can be added by grade, gender or individually. After saving, eligible students will be able to access the voting poll when it's open. 

The other reason why students might receive an warning message, is if they have previously voted.