Popper is not defined

Hi, I am getting the following error.

bootstrap.min.js?ver=3.3.6:6 Uncaught ReferenceError: Popper is not defined
at bootstrap.min.js?ver=3.3.6:6

I found a thread here but with no answer.
https://colorlibsupport.com/t/uncaught-referenceerror-popper-is-not-defined/27918

Do you have a solution for that?

Thanks