If you're referring to this:
If you have an unused worker, you may also build
a new farm or mine of any level. (Except for an Age II
mine – as we will see, you do not have 8 resources.)
in the example shown you only have 6 resources, the bit I quoted above is talking about building a new Age II mine costs 8 resources - as you only have 6 resources you can't afford it.