Sally has also been asked to help out in the lab when they are short-handed, doing quality assurance testing on the product.
That's not in her role description though, and requires a really different skill set.
With these two things in mind, the job description is neither fair or accurate — she can't do 2,000 bottles a day, doing quality assurance (QA) on the product is a different skill set to bottling, and doing QA on the product isn't mentioned in the job description.
With Sally’s agreement, her manager should rewrite the job description to realistically reflect her targets and tasks. Her performance is not the problem.
To build her skills and motivation, Sally’s manager should consider training her in the soda QA process.
This would also provide better long-term performance for the business.