“I can do that.”
But will you?
“I can do that” means you have the capacity, skills, knowledge, time, and resources to do this thing.
“I will do that” means you have the motivation, intention, desire, and drive to do it, as well as the capacity, skills, and so on.
I’d much rather hear someone say, “I will do that,” than “I can do that.”
“I will” leaves no doubt that the thing will get done. It’s a commitment to take action.
“I can” leaves a lot of room for doubt: will you or won’t you?
If you intend to do something, be clear: “I will do that.”
Otherwise, you risk creating confusion, frustration, and muddled expectations.