Can an individual legally create their own currency?

Yes. A private person can indeed create or invent a new currency and/or currency system. But to then be able to use it as a legal tender or a tradable exchangeable commodity would depend on the specific country they are in, and how the laws are structured by both the Government and/or the Financial Regulator.

You can certainly create it, whether or not you can use it - is an entirely different question.