I was able to resolve the problem.
The problem lies with the apache configuration. I substitute a component of the configuration and the 404 error cleared and REST API issue also resolved. I can create, save and update pages too.
$ sudo nano /etc/apache2/apache2.conf
<Directory "/var/www/html">
Options FollowSymLinks
AllowOverride All
Require all granted
</Directory>
Instead of AllowOverride All
, I had AllowOverride None
. I replaced and all problems solved.