I'm getting a page not found error when trying to visit my custom form
Problem 1: Incorrect namespace. The class name should not be included in the namespace. Your namespace should be:

Drupal\lookup\Form

Problem 2: The name of your class is not the same name as the routing file (capitalization).

Class name: lookupform

Routing file: _form: '\Drupal\lookup\Form\lookupForm'

(Capital F)

Problem 3 (credit: laymannx) - you're visiting the wrong path. Should be visiting /lookupform (after fixing the above problems).