。
She did not agree to the proposal.
Literal
She [topic-は] that proposal [to-に] didn't-agree.
提案 ('proposal, suggestion') is a fairly neutral noun, common in business, politics, and everyday planning. ~に同意する ('to agree to / consent to') takes its target with に — the thing being agreed to. The plain negative past 同意しなかった is straightforward, fitting a third-person narrative or report. Note that Japanese has a fine-grained vocabulary for kinds of agreement: 同意, 賛成, 承諾, 承認 — each with slightly different formal weight and contextual home.