The game rule is called randomTickSpeed not enableRandomTickSpeed :P

src/main/java/ganymedes01/etfuturum/configuration/configs/ConfigMixins.java

@@ -82,7 +82,7 @@ protected void syncConfigOptions() {
 
  enableNewElytraTakeoffLogic = getBoolean("enableNewElytraTakeoffLogic", catBackport, true, "When enabled, the 1.15+ elytra takeoff logic is used, when disabled, the 1.9-1.14 elytra takeoff logic is used.");
  enableDoWeatherCycle = getBoolean("enableDoWeatherCycle", catBackport, true, "Add the doWeatherCycle game rule from 1.11+");
- enableRandomTickSpeed = getBoolean("enableRandomTickSpeed", catBackport, true, "Add the enableRandomTickSpeed game rule from 1.8+");
+ enableRandomTickSpeed = getBoolean("enableRandomTickSpeed", catBackport, true, "Add the randomTickSpeed game rule from 1.8+");
  creativeFlightSpeedModifier = getFloat("creativeFlightSpeedModifier", catBackport, 2, 1, 4, "When greater than 1, boosts creative flight speed when sprinting, like in newer versions");
  bouncyBeds = getBoolean("bouncyBeds", catBackport, true, "Makes beds bouncy. Should work with most modded beds. For continuity disabling this also disables EFR beds being bouncy.\nModified Classes: net.minecraft.block.BlockBed");
  floorCeilingButtons = getBoolean("floorCeilingButtons", catBackport, true, "Allows ability to place buttons on the floor and ceiling. Note: Due to metadata limits, they won't rotate to face the player like how they were made to in more modern versions.\nModified Classes: net.minecraft.block.BlockButton");