Why can't I use @a or @p on the commandblocks on my Minecraft server on 1.13.2?

Why can't I use @a or @p on the commandblocks on my Minecraft server on 1.13.2? On my server I want to change the spawn of a jump and run with commandblocks, so with checkpoints. How can I now a) set the checkpoint for the player who triggers the block

and b) how can I generally targete only individual players with command blocks?

Did you insert the commands or write them in?