。
Yoko can't afford a computer.
Literal
Yoko [topic-は] computer [object-を] buy-cannot.
買えない is the potential negative of 買う — 'can't buy.' Used colloquially for 'can't afford.' コンピューター is the long-form katakana spelling (shorter: PC or パソコン).
。
Yoko can't afford a computer.