How to remove transparency of a keyed out object ( green screen )

Entirely depends on your keying settings and what else is going on. Since you haven't shared any eal info like a screenshot with the settings and how it looks in the comp we can't tell you much. If you're e.g. using Keylight it could be as easy as playing with the Alpha rollback values, if it's something else it could be necessary to add an adjustment effect like Levels to manipulate the Alpha/ transparency. Either way, you have to be more specific.

Mylenium

What effect are you using to remove the background? Even on professionally shot greenscreen footage, I have to fiddle with almost all of the options in Keylight to pull a really good key.

When you have a problem, select the problem layers in your timeline, press the "u" key twice to reveal all modified properties on the layers, then take a screenshot and drag it to the reply field on the forum so we can see what you are dealing with. Don't use the Drag & Drop here button; drag the screenshot directly to the reply field so you can see it, make it big, then describe your workflow and your problem. 

You might want to check out the User Guide.

After Effects is an incredibly complex app with a UI and a unique workflow.  YouTube is full of After Effects recipes prepared by enthusiasts that are based on poorly explained inefficient workflows. Vet your trainers...

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