In a single player world, try commands like

• /execute @p ~ ~ ~ spawnpoint

or

• /execute @p ~ ~ ~ setworldspawn

or these two

• summon Pig ~ ~ ~ {CustomName:Piggy}
• /execute @p ~ ~ ~ spreadplayers ~ ~ 0 1 false @e[name=Piggy]

Expected: the various execute commands succeed, and set the player's spawn point, or set the world spawn, or spread the pig, relative to the player.

Actual: command fails with an error message.