Heya,
if you are getting that error that just means BluePrism can't find what you are specifying it to look for. Once you have built the object to look for a browser and you tell it that you will attach to a running instance (this setting is in the application wizard). You create a navigate stage, give it the root element and choose attach option. When you do this you will get several criteria to help bp attach to the browser instance, these criteria are window title(collection and text string), child index and process name. play around with these to be as specific as you want or s vague as you want. sometimes more vagueness in detail can allow bp to find what you are trying to attach to.
search the knowledge base for more help, or try reading this page
https://blueprism.knowledgeowl.com/help/how-do-i-attach-to-a-version-of
this might shine some light for you
Dexter