If my android mobile tact key is not working Then how to lock my phone and Off my screen light?

For Any Android Phone

At First you have to open your Mobile "settings" > then go with "Accessibility" option > next choose "Interaction and dexterity" option > turn on Assistant Menu. 

After doing this you will get a flooting Button on your Smart phone Screen. You can set this button any where of screen as you would like to do, and you also get a screen off option Home button and recent button , easy screen short and more exsiting option at inside of flooting button.

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If I did any mistake, then tell the correct process in comment box, and if it's help you then say in comment box.
🙏Thank You🙏