Hello @sachin0a1,
I think you may have already followed the steps found here for changing WordPress file permissions: https://www.ads-software.com/support/article/changing-file-permissions/.
Some security plugins monitor your file permissions and secure your files and folders automatically. If you have a plugin like that, I recommend disabling it and testing whether you can change file permissions permanently.
If that doesn’t work, please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable the plugins one by one to identify the source of your troubles.
If you can install plugins, install and activate Health Check: https://www.ads-software.com/plugins/health-check/
It will add a new feature under Tools > Site Health.
On its Troubleshooting tab, you can Enable Troubleshooting Mode. This mode will disable all plugins, switch to a standard WordPress theme (if available), allow you to turn your plugins on and off, and switch between themes, without affecting normal visitors to your site. Following these steps allows you to test for various compatibility issues.
There’s a more detailed description about how to use the Health Check plugin and its Troubleshooting Mode at https://make.www.ads-software.com/support/handbook/appendix/troubleshooting-using-the-health-check/.
If none of the steps above help you, I recommend contacting your web host to help you with this issue. There may be some issue on your server that needs to be fixed.
