ERR_VENDOR_MISMATCH on QuickBooks: Vendor mismatch. Root cause: The vendor name or ID on the bill does not match any vendor in your QuickBooks chart of accounts. This occurs when you try to post a bill to a vendor that does not exist, or when the vendor name is spelled differently than it appears in your vendor list. QuickBooks enforces referential integrity — every bill must be linked to an existing vendor record to maintain data consistency. Step 1: Search for the vendor in QuickBooks. Go to Vendors > Vendor Center (or Vendors > Vendor List in older versions). Use the search box to search by name. Type partial names if unsure of spelling (e.g., "Acme" finds "Acme Corp", "Acme Inc"). If vendor appears, note exact name. Step 2: Check for duplicate vendor records. If search returns multiple similar names (e.g., "ABC Supplies" and "ABC Supply Co"), open each and compare contact info, address, phone to the bill. Determine correct vendor. If none match, vendor does not exist yet. Step 3: Create the vendor if it does not exist. If not in system, go to Vendors > New Vendor. Fill in vendor name exactly as on bill. Enter address, phone, email, tax ID if available. Click Save. QuickBooks assigns vendor ID. Use consistent naming (e.g., "Inc" not "Incorporated") to prevent mismatches. Step 4: Update the bill with the correct vendor. Go back to failed bill. Click Vendor field and select correct vendor from dropdown. If just created, it appears at bottom. Verify name matches exactly. Do not type manually — select from dropdown to ensure referential integrity. Step 5: Verify the bill amount and terms. After selecting vendor, confirm bill amount, due date, and terms (Net 30, Net 60, etc.) are correct. Check bill date matches source document from vendor. These details are vendor-specific and should match your agreement. Step 6: Save and post the bill. Click Save & Close. QuickBooks accepts it because vendor reference is valid. If batch-importing via CSV or API, use exact vendor names from Vendor Center. Most tools allow mapping vendor IDs instead of names — most reliable for bulk imports.