Assuming your stack is all 8.7, an elasticsearch output will, by default, create a data stream, not an index. If you want to create an index instead then set data_stream => false on the output.
If you do want a data stream then you cannot set the index name (you would use the various data_stream naming options).
Also, when writing to a data stream the output will not load a template, but I believe a template is required. You would have to load it through one of elasticsearch's template APIs.