Why can't I move a project to a specific workflow stage?

Typically, all transitions available for you are shown while clicking on the current "NEXT STAGE" bar.

If you expected to be able to transition a project to a different stage, and you do not see it, this could be related to three things:

1) You are trying to move too quickly ? . If you just moved a project, the system needs a few seconds to record the information. Try again in a few seconds. 

2) Your workflow does not have this transition created. Your admin should be able to check the workflow and confirm if that transition exists or not, and whether it is indeed needed.

3) There is a permission rule in place preventing you to see this transition. Your admin should be able to verify if there is a rule by which you/your user group are not allowed to see this movement, and edit if this is a mistake.