。
She regrets that she has never gone there even once.
Literal
She [topic-は] there [to-へ] even-once not-having-gone [nominalizer-の] [object-を] is-regretting.
The structure pivots on の nominalizing the entire embedded clause 一度も行ったことがない ('having never gone even once'), which then becomes the object of 後悔している ('is regretting') via を. 一度も + negative ない is the standard 'not even once' pattern — the も following an interrogative-like quantifier creates emphatic universal negation. 後悔している in the ~ている form expresses an ongoing emotional state rather than a completed act of regret — she is in the condition of regretting. Note that the natural particle here would be そこへ or そこに rather than そこの; the source phrasing reads as a slip, but the meaning is unambiguous.