mdname = r"D:Projectstest_map_colour.gdbGEBCO_Geotiff"
editmode = "EDIT_MOSAIC_DATASET"
editmethod = "INSERT"
funcfile = r"D:ProjectsStretched.rft.xml"
funcname = "#"

arcpy.EditRasterFunction_management(mdname, editmode, editmethod, funcfile, funcname)

Traceback (most recent call last):
  File "C:Program FilesJetBrainsPyCharm Community Edition 2019.1.1helperspydev_pydevd_bundlepydevd_exec.py", line 3, in Exec
    exec exp in global_vars, local_vars
  File "<input>", line 1, in <module>
  File "C:Program Files (x86)ArcGISDesktop10.6arcpyarcpygeoprocessing_base.py", line 510, in <lambda>
    return lambda *args: val(*gp_fixargs(args, True))
ExecuteError: ERROR 000582: Error occurred during execution.