South African Rand is ZAR, not SAR.
SAR is Saudi Arabian Riyal, I believe.
https://transferwise.com/help/article/2144226/rest-of-the-world-transfers/zar-transfers
TransferWise does seem to support ZAR natively in their app (but not SAR!). There may be restrictions though.
We don’t support either natively in Monzo.