speed optimize of wordpress

i have edited htaccess file for leverage browser caching in past which cause internal error.

maybe you made a typo in directives.
maybe your webserver software (Apache) did not loaded required modules:
mod_headers ( https://httpd.apache.org/docs/2.4/mod/mod_headers.html )
mod_expires ( https://httpd.apache.org/docs/2.4/mod/mod_expires.html )
mod_deflate ( https://httpd.apache.org/docs/2.4/mod/mod_deflate.html )

Maybe you are not allowed to AllowOverride All for your site folder ( https://httpd.apache.org/docs/2.4/mod/core.html#allowoverride )

If you are managing your server/vds by yourself – you have to check your webserver configuration (see docs links above), else – contact your webhosting support, so they can check this for you.

Related to caching plugin: 1) problems can occur because of your server config (see above)
2) static page caching does not necessarily speed up your site, it helps when your visitors are mostly anonymous and requesting same pages, for dynamic sites static page caching is not so efficient.

I’ll suggest you make a satging or clone of the main site & then work into the cloned site. This way you will not have any problem arising the unexpected behaviours for the main site. Once you are satisfied with the optimisation replace the main site by the optimised site.

how can i make clone of the main site?

thank you all for support