This article applies to Minecraft: Java Edition achievements only. For Minecraft (Bedrock) achievements information see: Minecraft Achievements FAQ
Can I Turn Achievements on and off in Realms?
If your Realm is set to Creative mode, achievements will be disabled for your world. Unfortunately, re-activating achievements is only possible if you have a backup from before Cheats or Creative mode was enabled. If no backups are available, then that world will remain without achievements enabled.
Achievements Not Unlocking
In Minecraft, achievements serve as an incentive to complete goals and milestones. These do not impact the gaming experience in any way. However, some players covet them because of the progress they represent.
Players may occasionally find that when they reach an objective, the achievement is not immediately unlocked. This can happen for a few reasons that vary based on gaming platform and version.
The following guidance will help troubleshoot Minecraft: Java Edition.
Minecraft: Java Edition
For Java versions 1.12 and higher, achievements are referred to as advancements. Advancements can represent different types of milestones such as normal game progression or challenges that do not require completion.
We recommend that you give the game 24 hours to display that the advancement has been completed.
If you still need assistance, contact Minecraft Support.