With the changes which Oracle made to Java licensing some time back Claris no-longer include a Java Runtime Engine (JRE) with the FileMaker server installer. With current versions of FileMaker Server this is only needed if you want to enable Web Publishing Engine (WPE).
When you go to enable the WPE the admin console provides you with to solutions to solve this issue. Most users will opt for using OpenJDK because it’s the most cost effective (there’s no price point better then free!) and simplest too.
Unfortunately OpenJDK have modified their website since those instructions were written and the ‘Download JRE’ button that used to exist, and which the instructions tell you to click, no-longer exists.
Follow the rest of the FileMaker server admin console instructions and you’re good to go!