Help! My shield isn't working!

You can't perfectly replicate shields at the moment.

Something you can do is:

Make a tool{your shield}.

Go to the "item in inventory tick"

add an "if" block and make the condition "if item in entity's off hand = {your shield}"

Then make it give you resistance or something like that.

As I said you can't perfectly replicate shields at the moment but this will give a defensive bonus.

Hopefully they will add a shield option to tools.

Change to "no additional trigger" and tie it to the "when tool in inventory tick" in your custom tool.

The world tick event you are using is if you wanted to make a custom weather or something like that.