"Untap" does not seem to be a commonly used word.
Most dictionaries I've looked at do not have an entry for a verb untap, although they do for the adjective untapped, which is actually an antonym of "unleashed." I think "untapped" is commonly used in the collocation "untapped potential." It's similar to how unopened exists as an adjective, but there is no corresponding verb "to unopen" (although unopen apparently has been used as an adjective meaning "closed").
So I agree that this is badly worded. A more usual way to phrase this would be "Tap into your computer's potential."
In general, the prefix un- seems to be attached more commonly to adjectives (including ones derived from past participles like "tapped" or "opened") than to verbs. Bea Bonmot's answer to the following question "Prefixes for Perishable" cites Ben Zimmer's article "The Un-Believable Un-Verb," which says that according to Yale linguist Laurence Horn, the verb prefix un- actually is derived in part from a different historical source, "the reversative prefix on(d)- (related to German ent- and Greek anti-)." Colin Fine's answer to the following question "'Unselect' or 'Deselect'?" says
Until quite recently, the "un-" prefix for verbs was pretty much
limited to what Whorf called the "cryptotype" of verbs to do with
fastening, wrapping, enclosing: "unlock, unwind, unwrap, unstrap,
unfasten, untie, unlatch, unroll" - there were a very few exceptions
like "unsay" (which is highly literary). [...] The adjectival prefix "un-" is a different matter, and has long been very widely applicable.
The meaning of the prefixed verb "to untap" (in the sense of "to unleash") fits in fairly well with the prefixed verbs above, but the meaning of the unprefixed verb tap does not fit in very well with the unprefixed verbs of "fastening, wrapping, enclosing."
The Oxford English Dictionary has an entry for the verb untap, but it is not defined and has only two examples, both from the seventeenth century:
1622 J. Mabbe tr. M. Alemán Rogue ii. 229 If I should suffer her
still to vntap my vessel, she would suck me dry at last.
1689 N. Lee
Princess of Cleve ii. iii, Does not your Politician,..after all his
Plotting, Drudging and Sweating at Lying, retire to some little Punk
and untap at Night?
It says it is derived from the verb tap and the senses 7 and 9 of the prefix un-:
7. With rare exceptions, the Old English verbs in un- are
transitive, and this has always remained the prevailing use. In Middle
English, however, intransitive uses of some common words are found, as
unbend, unclose, unlouk, and in casual formations as unbody. In the
later language the usage increases to some extent (as in unfold,
etc.), but is chiefly confined to words having some currency.
9. The redundant use of un- is rare, but occurs in Old
English unlíesan, and Middle English unloose, which has succeeded in
maintaining itself. Later instances are unbare, unsolve, unstrip
(16–17th cent.), and the modern dialect forms unempt(y), unrid, unthaw
(also locally uneave). Another redundant or extended use (= ‘peel
off’) exists in unpeel v.
So one existing verb with similar semantics to untap is unloose, which is basically synonymous to loosen.
There has already been a question on ELU about unpeel: "Is "unpeeling an orange" grammatically correct?" The following question about undust also seems relevant: "Dust vs. Undust?"
Also, here is a Language Log post with a lot of relevant links: "Why do thaw and unthaw mean the same thing?"