If you use the following search term on the Internet - apache post method not allowed url port 80 - you will find that there are a great many people with the same error, but with no solutions that have been posted.
This error appears to occur even when people use a .php file for the submit action, so this is clearly a configuration issue with the Apache web server at your hosting company.
In the posts I found, there was a lot of guessing and no one solved their problem. This either appears to be an Apache web server configuration issue (POST method is not enabled or not allowed in your web folder), or a folder/file permission issue (either your root web folder does not have the proper permissions to allow the POST method to execute) or the specific file does not have the proper permissions.
For any of these cases, your web hosting company should be familiar with the specific error and what to do to solve it.