Brian Helterline

The function definitions in the .cpp file should not have the default values specified. The compiler for the AVR boards is more permissive, but not the ESP compiler

It appears this issues has been fixed in the source code, but the .zip file that is used by the Library Manager is an outdated version of the code: https://downloads.arduino.cc/libraries/github.com/luisllamasbinaburo/InterpolationLib-1.0.0.zip...