Similar issue of connect PX4FLOW to Pixracer is discussed here: Pixracer with px4flow
To obtain data in Pixracer from PX4FLOW via I2C - you should connect PX4FLOW I2C to GPS of Pixracer (NOT telem 1/telem2).
There was also a problem with sonar data - after I soldered 5V power jumper on PX4FLOW and square jumper on sonar - sonar data also appeared in Pixracer.
After all, Pixracer does see all PX4FLOW data, but does not yet switch to POSCTL mode - will continue debugging…