I can't do that

…dekinai/ …dekimasen ; can not …./ is(/are) not able to …

sore/are : that

Watashi wa sore o dekimasen : I, that, cannot : I can not do that.

Above sentense doesn't include a verb but a object word.

Yesterday, I wrote in this blog about "can.."sentense, and explained that if a verb is included in the sentense, the verb should be conjugated.
The case of "cannot …"sentense is same.

hanasu : speak
hanas-eru : can speak
hanas-enai: can't speak
hanas-emasen : can't speak(polite)

Watashi wa nihongo o hanasemasen. : I, Japanese, can't speak: I can't speak Japanese.

And also explained a easier way to make " can..." sentense with changing a verb into a noun.

hanasu : speak
hanasu koto : speaking, to speak

Watashi wa nihongo o hanasukoto ga dekimasu.: I, Japanese, speaking,can: I can speak Japanese.

A negative sentense is same.

Watashi wa nihongo o hanasukoto ga dekimasen.; I, Japanese, speaking, can't : I can't speak Japanese.

You may already know a basic structure of easy Japanese sentense, then you can try to make sentenses as much as possible for your prctice.