If you want to Explode it, select it [single-click], then either pick the Explode icon in the Modify section of the Home ribbon, or type in EXPLODE.  Or do it in the other order [command first, then selection].  If it won't let you, the Block has been defined to not allow Exploding.  You can change that by calling up the BEDIT command, which you can do with a Block without Attributes by double-cliking on it -- maybe that's the issue, that one with Attributes doesn't work that way.  But you can select it [single-click], right-click and pick Block Editor.

 

If you don't have the Properties palette up already, type PROPERTIES.  In the Block category there is a line for "Allow exploding" with Yes and No choices.  But consider carefully whether there's a reason it was defined to not allow Exploding, because you're changing not just the selected Block but the Block definition -- if you change it from No to Yes, it won't just let you Explode that one, but the protection provided to all other Insertions of the same Block will be lost.

That Block category is at the bottom of the Properties palette.  You have to be in the Block Editor editing the Block in question, not just have an insertion of it selected in the generic Properties palette.

Have a Happy New Year!

Joseph, thank you anyway.