1×1 is used for non-standard ads. You cannot target it with standard display demand.
Step 1: Audit your ad units in the relevant PubGuru Header Bidding configuration
Step 2: Pay attention to the sizes configured per ad unit
Step 3: If there is a 1×1 size added where it’s not supposed to, delete it
Step 4: Save and publish your PubGuru Header Bidding configuration
Step 5: Also audit your Google Ad Manager ad unit settings (Inventory > Ad Units) and remove the 1×1 size if applicable