Hi there,
sorry for the issue you’re experiencing.
First, you have to be sure you have the privilege to add/edit posts on the site. In the admin dashboard – users – your profile, what’s your role?
In the case that you have all the privileges (site admin), etc.
The most common cause of this is an error in .htaccess file, plugin/theme conflict, using an outdated version of PHP, etc.
We’ll have to try one after the other to find the culprit.
1. Switch to (Activate) any of the default Twenty* themes (Twenty Twenty, etc.) and try again to see if the issue goes away.
2. Deactivate all your plugins and try again to see if the issues goes away. If it does go away, reactivate your plugin one by one while checking to find the culprit.
Meanwhile, on your WordPress Dashboard, go to Tools – Site Health, click on the info tab and click on Copy site info to clipboard. Paste the info you copied to pastebin.com and share the link here. It’ll provide some data that’ll help assist you in resolving the issue.