Minecraft – Can’t use all commands on a Minecraft PE world

For situations like this, I'd recommend running the command being executed itself (the /fill command) to see what the error is.

The main error appears to be that you're trying to fill a quarter of a million blocks, which is more the maximum (32678) that Minecraft allows for. You could reduce your fill region:

/fill ~-15 ~-15 ~-15 ~15 ~15 ~15 minecraft:diamond_block 0 replace minecraft:gold_block

Or, if the huge area is important, split it up into multiple commands:

/fill ~0 ~0 ~0 ~30 ~30 ~30 minecraft:diamond_block 0 replace minecraft:gold_block
/fill ~0 ~0 ~0 ~-30 ~30 ~30 minecraft:diamond_block 0 replace minecraft:gold_block
etc.

(I'd suggest not running either of these on a clock)

That should be it, but keep in mind that the command will also fail if there are no gold blocks to replace, or if you're too low-down/high-up so that the relative coordinate are selecting an area outside of the world.

If you have the command running on a repeating command block you would need to stop or break the command block and the time will continue as normal.

You can also use the gamerule doDaylightCycle command if it has been set to false (which stops time (day/night) from passing.

 /gamerule doDaylightCycle true