I was playing Scrabble recently and the word ail was on the table. I tried to add an -er to the word to make the word "ailer." My reasoning was that since ail is a verb, one could add a suffix to the word to describe "one who ails." The word was challenged and removed since it is apparently not a word.
A "jumper" is one that jumps. A "renter" is one that rents. A "maker" is one that makes. Why is "ailer" not someone that ails?
Scrabble's standard for what constitutes a word, as Janus Bahs Jacquet notes in a comment, is whether or not it appears in a dictionary. For organized play, specifically, the word must appear in an official dictionary currently published by Merriam-Webster; various other dictionaries have been used in the past. This is rather different from a writer's definition, or a grammarian or other linguist's definition of what constitutes a word.
No dictionary can contain every possible variation of every possible valid word. Modern dictionaries do attempt to include words and meanings as they are commonly used; there is a long editorial process of identifying new words, finding evidence for their meanings, and choosing whether or not to include them. But while ailer is superficially analogous to, say, wailer, it is not in common use. I did not turn up a single example in COCA or the BNC, and the only examples I found in Google Books after 1900 are names or transcription errors. Given publishing constraints, a word that may be understood in context, but is rarely ever encountered, may be excluded from dictionaries uncontroversially.
Lastly, as John Lawler alludes, the fact that jumper or maker exists does not require that ailer also be accepted universally. One may be a sufferer, but not really a convalescer. Or someone who was ailing and is recovering may be a survivor, which brings up the point that the same sound may be represented differently in the written word; see What’s the rule for adding “-er” vs. “-or” when nouning a verb?, Pedlar vs. peddler and other questions tagged as agent-noun-suffix. Perhaps the Official Scrabble Players Dictionary does list it— but as ailor. [Rhetorical point. It doesn't.]
Ail, like itch, cannot be turned into a noun by tacking on the -er suffix, because the verbs express 'being passively in a state', whereas the core idea of '-er' is 'engaged in, doing'.
P.S. But how about sufferer?
I think the occupational or vocational meaning of the -er suffix has been broadened to mean 'one who is a member of a group with a defining characteristic'. Consider the American neologism birther, i.e. "one who claims that President Obama was not born on US soil." Thus, a sufferer is not simply "*one who suffers" but one who is a member of a group with a particular disease, a gout-sufferer, a shingles-sufferer. We do not hear the word unaccompanied by the condition suffered.
Consider also that there is no ambiguity with the word freezer among native speakers. A freezer is an appliance that causes other things to become frozen, not "*a thing which becomes frozen".
