。
I concocted a story as my excuse for missing the party.
Literal
Party [obj-を] absent-did excuse [as-に], made-up-story [obj-を] concocted.
こねあげる is a compound verb 捏ねる 'to knead' + 上げる 'up' = 'to knead up / cook up / concoct (a story),' extending the kneading metaphor from dough to fabrication. 欠席した as a past verb phrase acts as a relative clause modifying 言い訳 'excuse.' The に on 言い訳 marks it as the role/purpose ('as an excuse for'). 作り話 is a compound of 作る 'make' + 話 'story' = 'made-up tale.'