@Ayushi Saxena 

try this and see if purchase reason is null

Is purchase_reason a choice value?

if yes and you want label then use this

current.purchase_reason.getDisplayValue()

(function executeRule(current, previous /*null when async*/) {
    function toTitleCase(str) {
        return str
            .toLowerCase()
            .replace(/[^a-zA-Z0-9 ]/g, '') // Remove special characters but keep spaces
            .replace(/\b\w/g, function(match) {
                return match.toUpperCase(); // Capitalize first letter of each word
            });
    }
    var purchaseReason = current.purchase_reason ? current.purchase_reason.toString() : '';
    var titleCaseReason = toTitleCase(purchaseReason);
    var originalShippingInstruction = current.u_shipping_instructions ? current.u_shipping_instructions.toString() : '';

    // Only update if purchase reason is present
    if (titleCaseReason) {
        current.u_shipping_instructions = titleCaseReason + ': ' + originalShippingInstruction;
    }
})(current, previous);

If my response helped please mark it correct and close the thread so that it benefits future readers.