Answer:
This is a comma splice.
↓
He shouldn't be operating a forklift right now, his certification expired last month.
Step-by-step explanation:
Any of these will fix the error.
> He shouldn't be operating a forklift right now. His certification expired last month.
> He shouldn't be operating a forklift right now; his certification expired last month.
> He shouldn't be operating a forklift right now because his certification expired last month.