In Paratext 8 and 9, PubAssist will only work on projects which have the status of “Publisher Edition” rather than “Translator Edition”. This status is reached after some months of vetting by the Paratext Regstry to make sure that another project isn’t already in that language by another sponsor, or if there is one, that the project sponsors are in agreement about both projects.
You can check the status by going to the Project Properties and clicking on the Paratext Registry link on the main page. Through the registry, you can request expedition of the vetting process if you need to do so.