Learn English – What do you call a pair of words which would be meaningless without one of them

I would call them hyphenated compounds, as opposed to solid compounds and open compounds. Note how they are not dashed. That's because a hyphen (-) is not the same as a dash (–, —, ⁓, ‒).

Short compounds may be written in three different ways, which do not correspond to different pronunciations, however:

• The "solid" or "closed" forms in which two usually moderately short words appear together as one. Solid compounds most likely consist of short (monosyllabic) units that often have been established in the language for a long time. Examples are housewife, lawsuit, wallpaper, etc.
• The hyphenated form in which two or more words are connected by a hyphen. Compounds that contain affixes, such as house-build(er) and single-mind(ed)(ness), as well as adjective-adjective compounds and verb-verb compounds, such as blue-green and freeze-dried, are often hyphenated. Compounds that contain articles, prepositions or conjunctions, such as rent-a-cop, mother-of-pearl and salt-and-pepper, are also often hyphenated.
• The open or spaced form consisting of newer combinations of usually longer words, such as distance learning, player piano, lawn tennis, etc.

It seems they are called Unpaired Words (maybe the best definition) or Absent Antonyms.

Unpaired Words
An unpaired word is one that, according to the usual rules of the language, would appear to have a related word but does not. Such words usually have a prefix or suffix that would imply that there is an antonym, with the prefix or suffix being absent or opposite. Many unpaired words are the result of one of the words disappearing from popular usage, though others were never part of a pairing and just begin with the same letters as used in common prefixes. The classification of a word as “unpaired” can be problematic, as a word thought to be unattested might reappear in real-world usage

You can find some examples here, but I'll list the majority of them here anyway for easy reference.

1. Words with no positive forms:
   Debunk; defenestrate; dejected; disconsolate; disdain; disgruntled; dishevelled; dismayed; disrupt; feckless; gormless; impetuous; impromptu; inane; incessant; inchoate; incognito; incommunicado; indomitable; ineffable; inept; inert; infernal; inhibited; insidious; insipid; insouciant; intact; invert; misgivings; misnomer; nonchalant; noncommittal; nondescript; nonpareil; nonplussed; unbeknownst; ungainly; unswerving; untold; untoward.

2. Words with uncommon positive forms:
   Disarray; disconcerting; immaculate; impeccable; inadvertent; incapacitated; incorrigible; inevitable; innocent; inscrutable; insensate; insufferable; interminable; unbridled; unflappable; unfurl; unkempt; unmitigated; unrequited; unruly; unthinkable; unwieldy.

3. Suffixes (asterisk means "word not existing"):

   • Reckless/*Reckful
   • Indefatigable/*defatigable -> fatigable
   • Flammable-Inflammable (not antonyms)