ERR_DISCOUNT_INVALID on QuickBooks: Discount invalid. Root cause: The discount code, early payment discount, or promotional rate applied is invalid. Discount may have expired, not apply to products on transaction, exceeded max uses, or restricted to specific customers. Step 1: Identify which discount was applied. Open bill or invoice that failed. Look at line items and bottom section where discounts applied. Note discount name, code, or percentage. Check discount line for rejection reason (e.g., "Expired", "Not applicable", "Limit exceeded"). If no reason shown, click field to see available options. Step 2: Check the discount expiration date. Go to Settings > Discounts. Search by name or code. Open and check "Expiration Date" or "Valid Until" field. If today is after expiration, discount no longer valid. If expired, remove from transaction or request manager extension or new code. Step 3: Verify discount applies to products on transaction. Check discount "Applicable Products" or "Restrictions" section. Some only apply to specific categories, services, or SKUs. Compare items on bill to eligible list. If items not eligible, cannot apply discount. Must remove discount or remove ineligible items. Step 4: Check if discount usage limit exceeded. In discount settings, look for "Usage Limit" or "Maximum Uses" field. If set (e.g., "5 times total"), check how many times already used. Go to Reports > Discount Usage to see history. If limit reached, cannot use until reset or new code created. Step 5: Verify discount applies to this customer. Check discount "Eligible Customers" or "Customer Restrictions" field. Some restricted to specific customers or segments. If restricted and current customer not on list, cannot apply. Contact manager to add customer to list or use different code for all customers. Step 6: Manually apply discount or request new code. If system discount invalid but manager approves, create manual discount line. Add line with negative amount (e.g., -$50 for $50 discount). Label clearly (e.g., "Manual Discount - Manager Approved"). Alternatively, request new code from manager not expired and applies to current transaction.