Old Venue 8 tablet. Missing keypad
Three options.


Use a USB-OTG adapter with a USB keyboard to unlock the device via password. Or a keyboard with a built-in OTG adapter.
Since you obviously have a Google Account, log into Android Device Manager, and wipe the device through there.
A Hard Reset via Recovery. Unlike some other Android devices, the Dell Venue 8 requires you press and hold Volume Down, press and hold power, then release power when you get the first logo. Do not release Volume Down until you see the Droidboot menu. Choose Recovery, and from there wipe data/cache and reboot. That's a Hard Reset, ALL your data will be erased.