Try doing it with different approach,
<?php if(osc_item_formated_price()=="Only Information Available" || $email == "OFF") { ?>
<button type="button" class="button-ii" disabled>Enquiry</button>
<?php } else { ?>
<button type="button" class="button-ii" data-toggle="modal" data-target="#myModal">Enquiry</button>
<?php } ?>
OR
<?php if(osc_item_formated_price()=="Only Information Available" || $email == "OFF") {
echo '<button type="button" class="button-ii" disabled>Enquiry</button>';
} else {
echo '<button type="button" class="button-ii" data-toggle="modal" data-target="#myModal">Enquiry</button>';
}
EDIT:
I also want to point out that you need to change disable=true to disabled.
And this line data-toggle="modal"disabled does not make sense, remove disabled after data-toggle="modal".
You also have an extra php opening tag here <?php>data-toggle="modal"disabled without a closing. You should remove that refer to the code above.