BA is not blocking this route.
T&Cs clearly state the use of the Amex voucher is subject to the usual redemption capacity restraints, so this is not mis-selling either.
The companion voucher does not “create” a newly available redemption seat. You have to work within the available redemption availability.
There is availability for two seats on the same flights outbound and return in Club during February, as I pointed out above; I know because I just did a dummy booking using my current voucher and could book these tickets fine.
I would suggest searching for available dates for two redemption seats in Club ITHOUT selecting your 2for1 voucher (out 4 Feb, back 14 Feb looks fine, with a reasonable stay in Cape Town out on a Friday, back on a Monday).
Once you know there is availability, then enter those exact dates, this time selecting the “I want to use my companion voucher” button.
If it still doesn’t work, then call up.
Indeed, for Gold card holders extra availability is made available in economy should you really need to get there on a day when there are no redemption seats available. THough very poor use of miles.
Have you tried the alternatives I set out above, such as getting availability on flights just to JNB using your 2for1 voucher?
South African routes have in no way been “blocked”,an routes are never available for regular redemption but not voucher redemption; if you are seeing this issue it is a tech error and you should call up and book over the phone, ensuring you don;t pay the extra fees.