What is an adjective for genuinely thinking oneself as skillful or knowing a lot about a particular subject but is actually inexperienced?

"All hat and no cattle", and if you want an equivalent that is (or was, anyways) perhaps better suited to females, "all talk and no cider". Also "much cry and little wool" is another. Yet one more that has recently been bent into this shape is "all talk and no walk". This last one is a pun on the earlier "all talk and no work" that has a different meaning. The new version borrows the meaning of "talks the talk but can't walk the walk".

There is a build-in sense of posturing or posing in each of these that isn't necessarily restricted to genuine belief, but does imply habitual behavior.

a victim of the Dunning-Kruger effect.

In the field of psychology, the Dunning–Kruger effect is a cognitive bias in which people of low ability have illusory superiority and mistakenly assess their cognitive ability as greater than it is (Wikipedia).