The president can order the operations of federal agencies, but they can’t order specific procedural outcomes.
The power of the DEA to schedule drugs comes from Congress, not from the executive branch. Congress created the DEA to build process to review drugs and manage them. The president is in charge of executing that procedure, not changing it or skipping it entirely. The power to effectively make laws is Congress, not the president.
As weird as it seems, there isn’t actually a loophole where the president can order someone to change the law even if that person is technically their employee.