"The only thing I didn't do was clean the floor, and that's because I couldn't
"I managed to do everything except clean the floor."
"The only thing I didn't do was clean the floor, and that's because I couldn't
"The only thing I didn't do." 「一つだけしなかったことは」
"was clean the floor" 「床掃除だった」
"thats because I couldn't." 「私の力不足で」
"I managed to do everything except clean the floor."
"I managed to do ~" 「〜できました」
"everything" 「[全部](https://eikaiwa.dmm.com/uknow/questions/62182/)」
"except ~" 「〜残って」・「〜以外」
The only thing I couldn't finish cleaning was the floor because I didn't have enough time.
I didn't have enough strength left to clean the kitchen floor but I did everything else.
この「力不足」がどう意味で使われているのかがわかりませんが、時間内に仕事を終えれなくて力不足だと言っているのなら、次のように言うと良いでしょう。
ーThe only thing I couldn't finish cleaning was the floor because I didn't have enough time.
「時間がなくて、床掃除だけができませんでした」
もし体力がないという意味の「力不足」で、床掃除ができなかったなら次のように言うと良いでしょう。
ーI didn't have enough strength left to clean the kitchen floor but I did everything else.
「力不足で床掃除ができませんでしたが、その他は全部やりました」
ご参考まで！
オーストラリア 
カナダ 
