The chosen Entity is not and Atomic entity or cannot be trimmed.

What do I do when I read this notification at the bottom of a page and want to trim etc. please?

A block insertion ( complex entity ) or dimension (another complex entity ) can't be trimmed.

First, explode it to convert it as single entities then trim

Ok. How do I do that?

Hello Jim

First of all click and select the block, then go to main menu - Modify and select the explode option then press enter, this should break down the block into single entities (lines, arcs etc.).
Try this and see how you go.