Good.
But:
TX has 19 million people right now over 18 who are citizens.
Which means 19 million people should be automatically registered to vote, like in most other countries.
There's still a gap of 3 million people, because only 16 million are registered.
Maybe not 3 million, if you account for prisoners who are not allowed to vote, but that's probably just a few hundred thousand.
You're correct, according to this Texas has ~250,000 residents in prisons (state, federal, local).
https://www.prisonpolicy.org/profiles/TX.html
Yeah, US-wide some 25-30 million citizens 18+ are not registered to vote (10-15% of the eligible) - which would be the case in almost every other civilized country.
Especially young people are not registered to vote, which is bizarre, because they are the group that you can most easily register to vote: at birth in the hospital, at school enrollment or when you get a driving licence.