Same. I'm trying to play games on scratch.mit.edu and even after I downoloaded Flash, it says that "Adobe Flash Player is missing". Weird, considering I saw one post being replied with "this question has been answered several times" and I've yet to see a solution.

Aye, it actually worked! Thanks!

So apparently, today I opened up my computer from sleep mode and decided to open up Opera, and suddenly I heard a bit of crackle sounds coming out from my MacBook Pro's battery.. and Opera crashed. It opened up again automatically, crashed again, opened up again, crashed again, opened up again, crashed again and stopped opening up.
Let me be more detailed on how it crashes and opens up back here:
I click on Opera on the dock bar, the icon bounces, and Opera opens up, but in 7-10 seconds, it crashes. It opens up automatically again, but crashes in the same amount of time as the last one. This process was done for like, 4 times, until it stopped opening up again. There was first a crash apology page on the 3rd time it opened (and crashed), but there was no option for crash reports, and when I tried opening (and crashing) Opera again, there wasn't any crash apology page. (Don't know if "crash apology page" is right, but I hope ya'll understand.)
I tried again in a few hours later, and it happened once again; without the crackle sounds. I uninstalled Opera by throwing it from the Applications folder to the trash can (the way to uninstall it, I think?), and went to Safari to install it again. That's when I noticed something peculiar; MacKeeper popup ads were popping up all of a sudden, and I didn't install anything.. and I'm guessing that this might had to do with Opera crashing. I reinstalled Opera back on my Mac and the problem remains the same. I removed adwares and malwares that I found with Malwarebytes, restarted the computer, tried opening up Opera again and the problem STILL remains the same.

Steps to reproduce: (Try to) Open up Opera from the dock bar.
Actual result: Crashed and opened up again for 4 times.
Expected result: Opera would've been up and running and I'd be able to do my job.

The OS I'm currently using is a MacOS Sierra, version 10.12.5. Since I reinstalled Opera from the Opera website, I'm predicting I'm using the latest version of it; I don't know what it is but I hope ya'll know. Didn't have enough time to open up Opera and check it's version in the 7-10 seconds I have before it crashes.

Hope ya'll can help! I think this might be relating to either the crackling sound from the hardware of my Mac, or the adware that I had on Safari. Thanks!