What is the usage of this resistor?
Both those resistors are pull-up resistors.  They exist so that the inputs to the CAN interface are at a known level ("idle high") for the period before the MCU (or other device) that is driving the CAN interface has initialized the ports it uses to communicate.

This is to stop any rogue noise getting onto the CAN bus while the MCU boots up and sets up its ports.

However, they are labelled "NA", and looking at an actual photograph of the board, you can see that R26 and R27 aren't populated.  My guess is that these exist as an option for using other chips with the same pinout as the MCP2551 which has an internal pullup resistor for the TX line, and really the RX line doesn't need one as such since it's an output.