Legacy Form qsID Error

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Hi,

I have created a Contact Form using Legacy Form. My form data are getting submitted, as I can see check that in reports, also I am receiving the successful submission mail.
But It is showing -
Oops, something is wrong with the form you posted (it doesn't have a question set id).

I have checked the hidden field value for qsID, and it has the value.
<input name="qsID" type="hidden" value="<?php  echo $qsID; ?>" />

But still it is showing the same error.

Please let me know why it is.!?

Thank you.