Which part of adding a tax to a product or a government investing in stuff are you saying is technically difficult?

I can't really think of anything related to this that anyone has ever said "even if the entire human race worked together on this for 3 decades we dont know how to achieve it" (actually, I've seen people say that a lot e.g. modern civilization isn't possible without fossil fuels because of EROEI, but those people are wrong and/or lying)