Form rejects multiple entries

I created a quiz on jotform 

3 people took it with no problem. But whenever the fourth person submit it it rejects with a message saying that multiple entries are not allowed 

What should I do to fix that problem 

Unfortunately, I cannot replicate the issue because your form is disabled.

It is possible you set a limit on submissions. You can set a limit based on the number of submissions, unique question or unique submission.

Please make sure that these options are not enabled in order to allow multiple entries.

Related guides:
How-to-Limit-Submissions-in-a-Form
How-to-set-Form-Limits-Based-on-a-Unique-Question
How-to-Set-Unique-Submissions-on-a-Form

Hope that helps. Thank you.