So, another issue my client faced which i helped her to solve. Client was trying to install a plugins in her wordpress website but cloudflare was blocking it. Client was using paid / pro cloudflare.
I checked cloudflare dashboard and found under Security > Security Rules page, at the bottom there is Cloudflare Managed Ruleset which was showing active ( screenshot below)
So i disable this option and got able to install the plugins successfully. After the plugins installed, i have Re-activated this option again.
All About Basic 