The available functions are POLY (the default) and SPLINE.
Note that the required maximum number of coefficients is different in each case. For POLY the maximum number of coefficients corresponds to the number of degrees plus one, so for a cubic use coeffs=4. For a SPLINE the maximum number of coefficients corresponds to 2x(knots+7), so for 3 knots use coeffs=20. If you are using SPLINE fits then you should ensure that the value of TUNE_MAXPOLY is set accordingly, as well as using an appropriate value for TRC_NPOLY.
Initial suggested value: POLY.