Problem with APKs all of a sudden on Kindle Fire?

All the apks I downloaded before yesterday worked. The "Install" button is impossible to press as of yesterday and today, as you can see in the photo. Please help!
Added (1). Yes, I tried other APKs and I've also tried shutting down my device. I checked my settings and "downloads from unknown sources" is turned on! This is weird…

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Is it good to power off and restart your Kindle Fire occasionally?

Is it good to power off and restart your Kindle Fire occasionally?

Read more: Is it good to power off and restart your Kindle Fire occasionally?

I bought the Micro Dr Card Adapter and still I do no know how to use it in the Kindle Fire Tanlet!

I bought the Micro Dr Card Adapter and still I do no know how to use it in the Kindle Fire Tanlet!

Read more: I bought the Micro Dr Card Adapter and still I do no know how to use it in the Kindle Fire Tanlet!