I've created a new Job Approval Process but can't post or amend vacancy

This will be linked to the business rules you have defined for your steps.

You define business rules for each step which apply when the step is active, and also, when the vacancy has been fully approved. 

You need to ensure that at some point within your process, you define a rule which allows you to post vacancies or update Job Requisitions;

This can be updated within your process, but again, be mindful, your changes will only apply to new vacancies.