。
I agreed to the plan unconditionally.
Literal
Unconditionally that plan [to-に] agreed.
同意する ('to agree, to consent') takes に for what is agreed to. 無条件で here uses で adverbially — 'in an unconditional way.' Both 無条件に and 無条件で exist; the に form is somewhat more adverbial-like, while で leans toward 'in the manner/condition of.'