If you have used your full Single Discretionary Allowance (SDA), you will not be able to make further ZAR to USD conversions that use your SDA until your allowance becomes available again in line with the applicable rules.
Please note: Your SDA allowance resets at the start of each calendar year. The annual limit applies from 1 January to 31 December, after which a new SDA allowance becomes available for the next calendar year.