According to VDO; error code 50 is due to a card fault. It could be down to one of several things;
you’ve put the card in the wrong way round 
the card has expired 
the card validity date has not been reached 
the chip in the card is damaged and the Vehicle Unit cannot read the data, the advice was try it in another vehicle if possible. If all else fails, declare it faulty and send it back to DVLA. If they find the chip faulty, they’ll free issue a replacement. Don’t forget to use Recorded Delivery and keep printouts whilst waiting for the new card 