problem

Please note - the items under DIAGNOSTICS in a pagespeed insights report are NOT errors, they are DIAGNOSTICS that show some advice.

If you want to remove Ensure text remains visible during webfont load diagnostic, then in your @font-face CSS add:

font-display: swap;

But this can lead to other issues such as CLS, so i personally would ignore that diagnostic.

how can i enable page caching on server

Ask your host if the provide server side page caching.
If not use one of the many Caching Plugins.