As a rider working under the P2P status you benefit from regulation that allows you to benefit from tax exemptions. As part of this status, your fees come directly from the customer, not Deliveroo.
For this reason, when restaurant partners choose to offer free delivery we unfortunately can't offer you those orders. You will of course still be eligible to receive any orders to customers who have paid a delivery fee.
Riders working under the self-employed status are eligible to receive free delivery orders. If you'd like to choose to switch to the self-employed status, and have access to all orders placed on Deliveroo, you can find out how to do this here.