My son's galaxy Tab A (model SM-T510) was giving the password error with the “try again in 30 sec" message, and it was suddenly asking for a password to shut down the tablet.
I had to hold the power button, vol up, and vol down all together for about 10 sec to power off. Once the Samsung screen starts blinking on and off, release the vol buttons and then the power button.
I originally tried the suggestion to use power and vol down, but bc all that does was take a ton of screenshots, so it was time to experiment! Lol