My first guess would be that you need to set USB or LSB mode. The second would be the PTT on the mic is not wired correctly, or the switch is broken. I would take a spare 3.5mm TRS (Tip Rind Sleeve) Plug and insert it into the Mic/PTT jack. Short the ring to sleeve connection and see if the TX light comes on then. If so, then the PTT mic switch or wiring is the issue.
Above are just guesses on my part. I have 3 v4 and 1 v5 but 0 v6 ubitx.