Hello. While I'm waiting for the Claris answering this question, maybe some community member has knowledge on this issue and it may be useful for others, too.

I bought a single licence of FileMaker Pro 19 but all my system is working in FileMaker 18. I need to downgrade version 19 into 18 and I could not find instructions for this. How to do it? Thanks

Are you experiencing issues with 19?  You should be able to use 19 with 18 as long as you don't add any 19 only features users on 18 will work just fine.

 

 

The FileMaker Server in use is still in version 15 and FileMaker 19 cannot access the solution. That's why I need to downgrade to 18. Thanks.

18 will not be able to connect to FileMaker Server 15 either. 

https://support.claris.com/s/article/System-Requirements-for-FileMaker-Pro-18-Advanced?language=en_US

About half way down are the versions it can connect to.

You are right. I need to confirm the version number of the FM Server. The Technical Staff of Claris can provide a new serial number for downgrade when they get the reasons to do so and what are the non-compatibility issues. Thank you for the help.