G29 S0 (or G29 without parameters) does not support a filename parameter. See https://duet3d.dozuki.com/Wiki/Gcode#Section_G29_Mesh_bed_probe. This facility was supported when G29 was first introduced into RRF, but was removed subsequently because it required additional RAM. Use G29 S0 followed by G29 S3 to probe and save the height map to a named file.