Did you check what the generated URL was in your Rule Logs?
I just tested with a sample variable with a request to HTTP Bin and it looks like the URL works as expected.
I can see in my rule logs that the URL is exactly what I would expect it to be.
I would make sure that the variable has exactly what you expect in it. No less. No more. In other words, you want to make sure there aren’t any spaces or any unexpected characters.
Looking back at your original post where you were asking about being able to inject a variable into a URL, it looks like the sample format string you provided has a space at the end of it, so you’ll want to make sure that doesn’t exist otherwise you’ll get a space in the resulting variable and that space will ultimately get included in the URL as well: