Getting an error on the TK website is something pretty normal. It's a hit or miss wheater the tool would work or not.
If it doesn't work, unfortunately, you need to call and as I mentioned before, no change fee should be collected.

Although another reminder that if you've booked with the 30% discount and you wish to rebook on/after 1 January 2021 or/and for travel on/after 16 June 2021 you would be asked for the mileage difference between the discounted and the original award rate. Be aware, if this is your case.