When you create a block using the dialog box, there's a check box called "Allow exploding" that is checked by default. If that was unchecked when the block was created, you'll get the message you've encountered.
You can open blocks like this in the Block Editor. In the properties of the Block Editor, you can change the Allow Exploding variable. Do that and then save on closing the Block Editor and you'll then be able to explode it.
HTH,
Trevor