Not capable at all would be non-verbal. This includes vocal and non-vocal verbal communication such as PECs and sign. Non-vocal is not speaking out loud. Able to speak but not doing so may be called selectively verbal.

Not trying at all could have several outcomes and reasons behavioral and medical and would not be any of these. They may not learn to read so lack literacy or may not learn properly how to speak and lack fluency in their first language.