Minecraft – replace Grass blocks in super flat to red sand

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.

Something you can do is:
A repeating command block, set to unconditional, always active, and with this command:

/execute at @e[type=minecraft:ender_pearl] run tp @p ~ ~ ~

Then have a chain command block run:

/execute at @e[type=minecraft:ender_pearl] run effect give @p resistance 2 255 true

I believe it is multiplayer friendly.
Make sure to type this command: /gamerule commandBlockOutput false, unless you want to have your chat spammed with tp messages.