Could you use the word "subestimate" when you want to say "underestimate"?
The general answer is, when you are making a choice between two words, and one of them is accepted by most speakers, and the other is not, it's better to use the accepted word, unless there is an important reason why that word won't work. That way you can avoid confusing your readers (or listeners), or causing them to doubt your knowledge of the English language.
"Subestimate" appears on some Web sites, but it is not entered in the most common dictionaries written for native English speakers, or in any learner's dictionaries. This suggests that "subestimate" is not a common word, and that many people will not be familiar with it.
I would use "underestimate." If you need to use "subestimate," find a way to define its meaning for your readers (or listeners).