403 Forbidden

I get this error message when I try to install the jetpack plug-in. I’ve followed all the troubleshooting tips on the developer site, any tips?:

The requested resource
/wp-admin/update.php
does not allow request data with POST requests, or the amount of data provided in the request exceeds the capacity limit.

Additionally, a 413 Request Entity Too Large error was encountered while trying to use an ErrorDocument to handle the request.

https://ww.wp.xz.cn/plugins/jetpack/