Possibly a typo in the ad unit name (learn more about ad units here). DFP could not attribute the impression.
Step 1: Audit the ad units on the page and correct any spelling errors. The ad unit names should match the ad unit code on DFP (on-page setup)
Step 2: Compare the Slot IDs in the configuration with your DFP settings. They should match your ad unit code (off-page setup)
MonetizeMore Header Bidding (M2HB) Config: