I get a message about missing sprite shaders. How do I fix this?

For PC:

This issue usually happens if your DirectX or video card graphics drivers are out of date, or if you are currently in a resolution that your computer does not fully support. First, ensure your DirectX and drivers are fully up to date.

Then, go into your Steam settings and make sure your Steam cloud is disabled. Shut down your Steam client entirely.

Next, please go to your userdata in Steam, which will probably be somewhere like this:
C:\Program Files (x86)\Steam\userdata

Then, search for a file called:
battleblocktheaterconfig

with all your hidden files shown in Windows (here's how to do this). 

Then, delete the file.

Try launching the game. If the game manages to boot, feel free to re-enable your Steam cloud.

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For Mac:

This issue usually happens if you are currently in a resolution that your computer does not fully support, or your video drivers are not up to date.

Selecting a lower resolution (such as 1280x800 retina) should allow you to start the game. Then you can select a higher resolution from within the game.