It says to allow it in "Security & Privacy". But when I click on "Allow" it doensn't do anything.
So I thought maybe I have to click on the "Click the lock to make changes." So I did, but then it asks for my account password. But when I try to type the password, it doesn't do anything. For some reason the password is not able to be typed in the password prompt.
You haven't said which version is your OS. I'm going to guess Mojave.
You're not quite doing it right. But the indication is quite easy to miss. (Thanks Apple!)
1. Uninstall BlueStacks.
2. Restart computer.
3. Reinstall Bluestacks.
4. When it says "Allow in Security & Privacy?", click on "Open Security Preferences"
5. You will see a new option in the bottom part of the window,
which is not usually there.
6. After clicking the "Allow" button you will see a list of software/drivers that have been blocked. Tick the checkbox next to the appropriate items for the software and/or device you have just installed and then click OK and restart the computer.
You should be fine now. At some point, you'll be asked for your password. Enter it as necessary.
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Be aware that newly installed software and/or drivers will only appear in the list for
30 minutes after the driver/software in question has been installed. If the Allow button doesn’t show or the driver you wish to load isn't showing up on the following screen, you will need to reinstall the software again and restart the computer. Once your computer has restarted go straight to System Preferences > Security and Privacy and the option to "Allow" the driver/software to load should appear.
If software from multiple developers have been installed, the System & Privacy pane may display a more generic message that reads "some system software was blocked from loading." In this case, after clicking Allow a list will appear and the software from each developer can be allowed individually.
If you are still unable to see the "Allow" prompt to appear, try creating a new Administrator account then follow these instructions again.