When a new workflow is created it shows a blank page like the image shown below.
Most Probable Cause
This is usually caused by the [wf_workflow_version] Submit UI action and the [global] Submit UI action being customized.
Go to UI actions > Filter Name = Submit, Table = wf_workflow_version OR Table = Global
Once you open the wf_workflow_version Submit UI action, you will see that it is referring to the Global Submit UI action. Verify that both [Global] and [wf_workflow_version] Submit UI actions are active.
If the above two UI actions are customised, try to revert to out-of-box (OOB) and see if it works.