English Translation
Illness prevented him from attending the gathering.
でられなかった should be spelled in Kanji 出られなかった. 出る is registered as one of the easiest Kanji, and ranked as 漢字検定10級レベル. If you spell 病 in Kanji (ranked 漢字検定8級レベル), 出 also should be in Kanji in the same sentence.