This message tells that your Test Object is looking at a HTML element which you expect to select by a XPath
//div[@class = 'toast-item toast-type-error' and (text() = 'Stockroom for this name already exists' or .= 'Stockroom for this name already exists')]
Please check if this xpath is correct or not.
You need to look at the HTML source around the element you want to select. You may want to learn how to use Browser’s DevTool.
Please take the screenshot of the HTML source and share it to us.