Sometimes I tell in my manuscript instead of showing, all novelists do. The trick is to know when to bypass description, dialogue, and narrative and just show to keep the proper pacing, and here's a hint: it's not often. But one place you should never tell is in your query and pitch. But the show-don't-tell idea is a tad different for these items than your manuscript (naturally you are telling your story in your query and you don't have all the tools to show that you have in a novel). What I'm talking about is telling an agent what the tone of your story is instead of showing them.
For instance saying "TITLE is a fast-paced thriller" when the synopsis of your story reads like a road map to boringville.
Or saying "readers will laugh out loud" when your query is dry as day-old bread. Make us laugh!
Telling your reader these things undermines your query in two ways.
1) It makes a promise that your query better deliver
2) It shows your not confidant in your pitch's writing
So cozy up to the delete and start showing what your novel can do.