This is an excuse not to text you back, but with good reason. If he's genuinely bad at conveying sarcasm via text so hard or if his texting tone has started fights in the past, he's probably trying to avoid conflict. If your guy is saying something like this, but normally will text you all day, every day, pay attention. If he's amazing text conversation online dating reddit that he doesn't like texting, but is constantly on the phone when you are together, pay attention to his behavior.
He may not be outright lying, but his excuse for not texting you seems less than legitimate. If "being bad at texting" becomes a regular excuse for not replying to you, you have a right to bring it up. Ask your boo why he thinks he's bad at texting. However, if you are still having doubts about your man's lack of text-versation, ask yourself if he's showing up for you in real life.