"Lizard brain" is part of the Sri Executive Model, referring to the contrarian and lazy part of me that automatically resists the thought of doing anything that isn't a big immediate reward. Doing nothing is preferable. I try to avoid falling into the trap of negotiating with the lizard brain, since it doesn't negotiate in good faith.
In personal ADHD terms, I suspect the Lizard Brain might be the effect of low dopamine in the brain.
In personal GAD terms, it could be genuine anxiety about doing something, especially if I am not confident about meeting the expectations that are in place.
In personal ASD terms, expectations that are not clear may be another source of resistance or emotional stress.