I want to add a custom checkbox just above the place order button. The user should be allowed to click the Place Order button only after the checkbox is checked. Thanks in advance!
Magento have in-built functionality to set checkbox above place Order Button.
Login to Backend :
Menu->Stores->Configuration->Sales->Checkout->->Checkout Options Under that form change the value of Enable Terms and Conditions to YES.
After that run php bin/magento c:f command & see the Magic.