An adjective that is a combination of car- “do, make” with the suffix -ima “-able”, so literally “able to be done, ✱doable”, with alternate meanings like “feasible, possible” (PE22/137, 155). As a verbal formation, the á is long in cárima. In one place Tolkien had carima with a short a as an ordinary adjective replacing the passive participle carina “done” (VT43/15), but that seems to have been a transient idea. In another place Tolkien glossed ᴹQ. kárima as “{able to make (likely, apt to) >>} able to be made” (PE22/111). I believe both meanings of cárima are applicable, depending on context: hirië Valinor cárima (ná) “finding Valinor (is) doable” vs. macili cárimë anganen “swords [are] makable with iron”.
Quenya
lacárima
adjective. not feasible, impossible
cárima
adjective. feasible, possible (to do)
cárima
adjective. feasible, possible (to do), able to be done, feasible, possible (to do), able to be done, [ᴹQ.] able to be made
lacárima
adjective. not feasible, impossible
A negation of cárima “feasible, possible”, first appearing as ᴹQ. alkárima “impossible to make” in the Quenya Verbal System (QVS) of the 1940s (PE22/111), then as Q. la-kárima “not feasible, impossible” in Late Notes on Verb Structure (LVS) from around 1969 (PE22/153). The 1969 variant appeared in a paragraph which started “As a prefix the reduced form la (short a) appears. This is used with verbal derivatives, nouns or adjectives ...”. Tolkien rejected this paragraph, but the new version of the paragraph read “With nouns and adjs. (not regularly attached to verbal stems, such as -ima, -ite) the form was al before vowels and usually (in all later and free formations) before initial consonants that in Q. were permitted after l”. Thus in the revised paradigm, lacárima remained valid since it was based on a verbal stem.
Neo-Quenya: For purposes of Neo-Quenya, I would keep ᴺQ. lacárima “not feasible, impossible”.