Open Settings
Tap Display & Brightness
Tap Auto-Lock
Select Never
If Auto-Lock is unavailable, check Low Power Mode and turn it off
Keep the iPad plugged in if needed for long use
Use Guided Access if you want to keep one app open without sleeping
Go to Settings
Tap Accessibility
Tap Guided Access
Turn on Guided Access
Open the app you want to keep active
Triple-click the top button or Home button
Tap Start