Okay, basically it was a bug. Odoo Enterprise Version 14.0. When you install the Website Module and uninstall it the above-mentioned error comes in. But if you install any other new app (in my case, I have installed Notes), the system gets refreshed automatically and the problem is solved.

Hi,

It seems there is some changes in your security groups, each application in Odoo has specific groups and it can be shown in groups like (Sales / User: Own Documents Only)

So, you need to check the security groups for these apps,

 Go to groups after enabling developer mode and from settings -> Users & Companies -> Group and from "Group by" select Application:

It should be look like this unless you have added custom groups. 

If the sales app not showing, that's mean someone remove the application from the group, so you need to fix them by open the group and set the application again.