。
She couldn't have been happier.
Literal
She [topic-は] this-on top-not-existing happy was.
この上なく is a literary-leaning superlative — literally 'with no more above this,' meaning 'unsurpassably, supremely.' Built from この上 ('on top of this') + the negative なく ('without'). It scopes the following adjective at its absolute maximum — happiness with no upper bound. The whole phrase has a written, slightly elegant feel, often appearing in narrative prose or reflective writing. Plain 幸せ would be everyday; この上なく幸せ adds a hush of awe, the kind of phrasing used to describe a peak emotional moment.