Can't activate Preview mode Android Studio
Had the same problem, this solution worked for me File - Invalidate Caches/Restart
I had the same problem.Then I chose a different device.I saw the error window with the ability to click "rebuild".I clicked and it worked.
Studio 2.2.1 / Build #AI-143.2821654, April 28, 2016.
Regarding: Missing the Preview Tab or grayed out Designer from View/Tool.
From the directory tree, click on the following directory to access activity_main.xml:
app/src/main/res/layout/activity_main.xmlor (depending on your directory structure, actual location may vary)
app/res/layout/activity_main.xmlAfterward, you should notice the Preview tab to the right.
To access Designer. You'll be able to find the Design|Text tab below your code.
P.S. My problem was the Designer was grayed out. After the fix, the Designer from Menu bar/View/Tool Window/Designer is still grayed out. But you should be able to access the designer anyway.
