The good or bad news will accompany you telling me what computer operating system you are running your Premiere Elements 12 on.
Good news - if you have Windows....then Expert workspace/fx Effects tab at bottom of interface/Keying and under Keying you will find the Green Screen Key and more.
Bad news - if you have Mac....Mac does not come with ChromaKey, Green Screen Key, and Blue Screen Key. There are some who suggest Videomerge as a substitute (not one of my favorites). If you are a Mac user with Premiere Elements 12, please check out my blog post relating to the Difference Matte Key as substitute for the Green Screen Key in Premiere Elements Mac.
Difference Matte can also be used in Premiere Elements Windows.
Please review and then let us know - good news or bad news.
And if ATR's Difference Matte substitute doesn't work for you then, depending on where you bought it, you may be eligible for Adobe's no reason required thirty day money back guarantee. Don't let that thirty day window pass.
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If this post or another user's post resolves the original issue, please mark the posts as correct and/or helpful accordingly. This helps other users with similar trouble get answers to their questions quicker. Thanks.
Sorry, but I've got to give props to Videomerge.
It's not as powerful as Chroma Key, to be sure -- but, with a well-shot key, it can certainly do that job at least as well.
I hope that you respond that you have Premiere Elements 12 on Windows and your question evolved because you needed to know the location of Green Screen Key in one of its two workspaces Therein would reside the basis of your question..
In Premiere Elements 12 Windows, the Keying Effects are found in the Expert workspace not the Quick workspace. So, there is a good possibility that you were looking for Keying/Green Screen Key and other keying effects in the Quick workspace.
Again, Premiere Elements 12 Windows Expert workspace - fx Effects Tab at bottom of Expert workspace/Keying/Green Screen Key and other.
(Alternate route to same place = Action Bar Menu at top of Expert workspace/Effects.)
But, if you are a Premiere Elements 12 Mac user, not to worry. There is always the Difference Matte as explained in my original post numbered 1.
Looking forward to your results.