NAN value-problem

Hello!
I make one project but instead of the measured SOP2 value, NAN appears on the display. I really don’t know why, where the problem comes from.
What needs to be changed in the code?
I really need help, please!
Link for the code:
https://create.arduino.cc/editor/scarlatmd/e6851e31-06de-499c-9f12-6674c337c9e5/preview

3 posts were merged into an existing topic: NAN value a BIG problem