Is a company prohibited from being the appointor of a discretionary trust?
No, that's fine. Unusual, but fine. An appointor arrangement available online is to have a company appointor and a nominated backup appointor (an individual) to step in if the company cannot act. In this situation the company appointor acts until it is wound up/deregistered/cannot act as appointor etc. and is then replaced by appointor two (commonly a director of the company).
A company can act as appointor without a nominated backup if you wish (so long as that company is not the trustee of the trust), however you will need to call for assistance when placing your order online.