I have a 2018 FLTRXS that does not have connection with the SEPST program using SE pro super Tuner 32109-08A. There can be connection with newer VCI 32109-08C. The 08A version is making connection with the older bikes, I tested 2008 and 2012 touring bikes, which have 4-pin connector.
Can there be a programmed block preventing 32109-08A connection to M8 engine? 6-PIN cable is good. Both modules are having the latest firmware (414). 32109-08A is never paired with an ECM.
If the 32108-08A is one of those refurbished ones where they have reset the flash then it will not work with a 6 pin bike no matter what you do.
If however it is just NOS (new old stock) and has never been used then it should work but I don't have a 2018 bike here to test but I have had them hooked up to a 2017