Expanding @theadaloguy’s reply - you can’t add several records using Adalo API “POST” method as it is not supported by API.
Moreover, there is an error in your JSON - you need to have a comma after the first section. Tools like https://jsonlint.com/ could be very useful in JSON validation. But even if you corrected this - see above
Another question is why would you want to add the records using a custom action instead of adding it using regular way, but I can imagine some cases when this is required