It sounds like you are getting the run around by your strata manager (SM).
Sure the Act says:
36 Requisition for motion to be included on agenda for general meeting
(1) Any person entitled to vote at a general meeting of an owners corporation on a motion that does not require a unanimous resolution may, by notice in writing served on the secretary of the executive committee, require inclusion in the agenda of the next general meeting of the owners corporation of a motion set out in the notice and the secretary must comply with the notice.
But your SM is getting very picky if requiring this because if the SM is preparing and distributing the agenda then the SM is doing the role of the secretary, permission for this would be in the agreement, and serving the notice of your motion on the SM is serving it on “the secretary” if the SM is engaged to be the secretary. It is a function of the Secretary to give EGM notices, so if the SM is acting as the Secretary and giving the notices then giving the motion to the SM should be adequate. I think you are getting mucked around.
It sounds like there has been an addition to the common property without OC approval. The EC could send the person responsible a letter requesting the shed be removed or the EC will commence action through mediation and then through the CTTT if necessary. Is there a motion to approve the shed on the agenda?
Do not forget the EC does not need the SMs permission to take action and that the SM is subordinate to the EC. If you have the support of the EC then the EC can take action if it decides it wants to.