Well I always clear all opportunities, including the WA ones or, if an opportunity is really important I will get the sim to complete it and then try the gardening one again, it has always been the case for me that somehow other opportunities inhibit the list, if I make any sense, once opportunities are cleared, the list is restored to full so you can choose which opportunity to give your sim.
Besides, you can always go to Egypt and there give your sim the cancelled opportunity via the same Nraas cheat. I do it all the time >:)