Twan SRLGGB2L is a british BIC code. You can easily see that from the GB in there (digits 5-6). This isn't a South African bank account. A real South African BIC has ZA as digits 5-6.
bunq does not support sending South African Rand to bank accounts in countries other than South Africa itself. Also, this is the BIC from Starling Bank and they only offer GBP, EUR and USD accounts at the moment.