To reproduce, look at the command syntax help for /clear:

1. Type in /clear
2. Hit the space bar
3. Hit esc
4. You will see the command syntax help for /clear: /clear [<targets>] [<item>] (see attached screenshot)

The provided syntax suggests that

/clear minecraft:enchanted_book{}

is valid, which it isn't.

The correct syntax help for this example should be /clear [<targets> [<item>]].