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#11
Depends on the policy set up but PIN certainly don't have to be just numeric. They can be punctuation, letters or a mixture.
You can see this tutorial for setting up different lengths, alpha/numeric requirements etc if you are interested.
Specify Maximum and Minimum PIN Length in Windows 10 | Tutorials
Your problem may have been wrong keyboard layout was defaulted or num-lock was on. I often get PIN wrong as French keyboard you press shift + 1 to get a number one and on English you just press 1.