Which is your version of KiCAD?
It seems that recently has been added a change in kicad_pcb file format…
Double quotes has been added to the internal layer description…
This seems to break the STEP exporter…
comparison example:
OLD standard file format:
(layers
(0 F.Cu signal)
(31 B.Cu signal)
(32 B.Adhes user)
(33 F.Adhes user)
(34 B.Paste user)
(35 F.Paste user)
(36 B.SilkS user)
(37 F.SilkS user)
(38 B.Mask user)
(39 F.Mask user)
(40 Dwgs.User user)
(41 Cmts.User user)
(42 Eco1.User user)
(43 Eco2.User user)
(44 Edge.Cuts user)
(45 Margin user)
(46 B.CrtYd user)
(47 F.CrtYd user)
(48 B.Fab user)
(49 F.Fab user)
)
and the format in your file:
(layers
(0 "F.Cu" signal)
(31 "B.Cu" signal)
(32 "B.Adhes" user)
(33 "F.Adhes" user)
(34 "B.Paste" user)
(35 "F.Paste" user)
(36 "B.SilkS" user)
(37 "F.SilkS" user)
(38 "B.Mask" user)
(39 "F.Mask" user)
(40 "Dwgs.User" user)
(41 "Cmts.User" user)
(42 "Eco1.User" user)
(43 "Eco2.User" user)
(44 "Edge.Cuts" user)
(45 "Margin" user)
(46 "B.CrtYd" user)
(47 "F.CrtYd" user)
(48 "B.Fab" user)
(49 "F.Fab" user)
)