Posted by sp1der on Jan 29, 2014; 9:47am
URL: https://forum.librecad.org/The-chosen-Entity-is-not-and-Atomic-entity-or-cannot-be-trimmed-tp5709441p5709455.html

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.