Reason:
This happens because the app was downloaded using another Apple ID and xcode is tied to that account.
How to solve this ?
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Sign in into Mac App Store with “your current ID account” and check under Purchases menu, whether you have xcode.
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Also check in System Preferences » App Store » What ID shows below “Automatic download apps purchased…”
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If xcode is present under your current ID Purchases (grayed out saying INSTALLED), feel free to erase the app from your HD. An install button must be shown after and you can download and update the app again.
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If the above doesn’t work, you can get rid of the greyed out Apple ID email by making following selection
Applications » xcode (right click on Show Package Contents) » Contents » and delete the file inside “_MASReceipt” folder (do a backup before just in case).
xcode will still work, you must get rid of the grayed login name.This will work if xcode is already in your curent Apple ID