When I use GREP to replace multiple spaces with a single space in InDesign, it says ‘cannot match.’

Hi @hibru305572725zfd:

That particular GREP query is looking for multiple spaces that appear in any combination and you aren't showing any. Instead, we see single spacebar spaces (the dots) and seemingly random single em spaces (the dot with the line underneath).

Guessing that you are trying to convert them to regular spacebar spaces, you can do this with a regular text Find/Change:

Since spaces don't show up in a text box, I highlighted the space added to the Change to field. Once you convert all of the em spaces to spacebar spaces, you might want to run the Remove Trailing White Space query—there is at least one em space at the end of a paragraph visible. 

~Barb