If you are talking about RTR bit in ARBITRATION FIELD of CAN frame, The polarity of the RTR bit indicates whether a transmitted frame is a DATA FRAME(RTR bit ’dominant’) or a REMOTE FRAME (RTR bit ’recessive’).
You are right, without detect of STLINKV2 on PC side the FW is not able to be upgraded...in this case, may be you can try CubeProgrammer instead of STLINK utility?
Normally the VCP is supported by STLINKV2 embedded on ST STM32 boards. is this issue discovered on ST board? or which ST board you are working on? thanks!