Due to some security and reliability concerns, System Administrator rights are restricted to Atlassians only. Specific customers for a particular instance can be granted "Administrator" privileges which are sufficient for them to manage its content.
Hope that clarifies your concern.
If you're missing things in OnDemand, then I'm afraid you are stuck - As Abdoulaye has said, OnDemand has limitations that the downloaded products do not. Atlassian are working on widening the range of things you can do as an owner.
You need to talk to OnDemand support to do what you need, or (as I'm not 100% clear what access you have) get them to add you to all the groups that you might be missing.
You may want to have at the below links regarding plugins installation restrictions:
We are sorry for any inconvenience.
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