The system is requiring you to update your AdX Run of Network line item to target or add the specified ad unit sizes.
Step 1: Check if the sizes mentioned are relevant to your publisher. You can dismiss the warning/error if your publisher is not running them and stop at this point.
Step 2: Open the line item in question to edit.
Step 3: Add the inventory size/s in the targeting.
Step 4: Save
Step 5: Click the Creatives tab
Step 6: Open each creative and add the sizes/s you added in the line item settings.
Step 7: Save