I have developed a message extension bot using Bot framework v3. It lists user profiles in the form of thumbnail card which gets data from SharePoint list. But when the bot is getting response after 15s it gives error as "Unable to reach the app". So it is possible to modify text.
That message is coming from MS Teams whenever the messaging extension is unable to either reach your bot at all or if you bot returns an error response like 500 or 401/403.
Make sure your bot is running properly with the correct bot id (has to be the same id you use in the manifest) and make sure it sends a 200 and the expected payload. If there's a mismatch of botid in manifest and the one you use when starting your bot, you also might get that error (it would show a 403)
You can not change that message yourself.
You might see some errors for the bot in dev.botframework.com if you registered your bot there.