Most common questions used to investigate
Is your dough sticky or wet?
Are you over working your cookie dough?
Did you use cold butter?
Is the temperature in your oven too high?
Common conclusions
There isn't enough flower in your dough and that makes your cookies tough.
Over working your dough will cause your cookies to be hard.
Cold butter is difficult to cream and that can make your cookies dense. use cool butter but not cold butter, leave the butter out for fifteen minutes before using but don't leave the butter out for too long.
A hot oven could cause your cookies to dry out and harden. Use a thermometer to make sure your oven temperature is correct.
Sorry, we were unable to find the reason for your hard cookies.
References
https://www.mnn.com/food/recipes/stories/whats-wrong-with-my-cookies-a-troubleshooting-guide
https://www.thespruceeats.com/common-cookie-baking-mistakes-4137054
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