You can access the session attributes using sessionScope in your JSP.
So you need to call like this: ${sessionScope.discountAdded == 'true'}
<c:choose>
<c:when test="${sessionScope.discountAdded == 'true'}">
<p align="center">
All Discount(s) added successfully!
</p>
</c:when>
<c:when test="${sessionScope.discountAdded == 'false'}">
<p align="center">
Error Found! No Discounts added!
</p>
</c:when>
</c:choose>
Suggestion:
If discountAdded is going to contain only boolean value, then instead of <c:when>
conditions, use <c:otherwise>
for the another condition.
<c:choose>
<c:when test="${sessionScope.discountAdded == 'true'}">
<p align="center">
All Discount(s) added successfully!
</p>
</c:when>
<c:otherwise>
<p align="center">
Error Found! No Discounts added!
</p>
</c:otherwise>
</c:choose>