よんどころない事情で、今年の夏は別荘で過ごせないんだ。

、。

Due to unavoidable circumstances, I can't spend this summer at the villa.

Literal

Unavoidable circumstances [with-で], this-year [possessive-の] summer [topic-は] villa [at-で] can't-spend [casual-んだ].

よんどころない ('unavoidable, can't be helped') is a somewhat literary adjective — the same root as やむを得ない but with a more formal, old-fashioned flavor. 別荘 ('villa, vacation home') implies affluence. 過ごせない is the negative potential of 過ごす 'to spend (time).' The speaker regrets missing their usual summer retreat.