Hello
As you can see from the docs
docs.flashphoner.com
CentOS 7.6
CentOS 7.8
CentOS 8
CentOS Stream
where tested
Regarding Centos 9. We didn't test.
It should work. There is no show stoppers.
You can download and install the
Trial license on Centos 9 server to be sure it is working.
Once it is confirmed, you will be able to migrate production instance to Centos 9.
If Trial license already exist, you can request extension to
sales@flashphoner.com