No code, no schematic, I don't know what a "de0nano" is, and I don't have my crystal ball handy.
Wild guess: You are attempting to use SoftwareSerial on the hardware serial pins.
No code, no schematic, I don't know what a "de0nano" is, and I don't have my crystal ball handy.
Wild guess: You are attempting to use SoftwareSerial on the hardware serial pins.