Hi,
Haven't done a clean installation for 1.9.11 yet nor ever used Azure Web App platform, but based on experience maybe you got the package from the wrong source. So from where did you got the osTicket installation package?Official download page btw. is the following: http://osticket.com/download
Yes, I downloaded from the official download page.The file downloaded was osTicket-v1.9.11.zip.
I was able to get v1.10-rc.2 successfully installed, although I need more testing before I can be 100% sure all is OK with it. I also tried installing v1.9.12, the other latest release, but encountered the same error.