Not sure why you cannot install 1.30.2. That should not be happening.
However, go ahead and install 1.29.2.6342 from Package Center. It is a stable, solid release.
If you need a feature/bug fix in 1.30.x, you can then try upgrading. See release announcement thread linked below.
Pay attention to the messages about file permissions during the installation process.
Useful Information:
Installation & Setup of Plex Media Server on Synology
Synology FAQ - Questions, Answers, and a few How-To’s
