Unable to install a package: Error while downloading packages metainfo: No package(s) available to install

Symptoms

An attempt to Plesk or one of its packages fails with the following error:

Error while downloading packages metainfo: No package(s) available to install. Trying again.
Failed to install mod_python:
Number of retries is exceeded.
InstallError: No package(s) available to install

ERROR: The Yum utility failed to install the required packages.
Attention! Your software might be inoperable.

The following error can be observed in the /var/log/plesk/install/autoinstaller3.log

Failed to install mod_python:
Error while downloading packages metainfo: No package(s) available to install. Trying again.
Failed to install mod_python:
Number of retries is exceeded.
Traceback (most recent call last):
  File "/usr/local/psa/bin/yum_install", line 225, in <module>
    main()
  File "/usr/local/psa/bin/yum_install", line 211, in main
    inst, rem = installer.resolve(to_install, opts.remove, opts.tries)
  File "/usr/local/psa/bin/yum_install", line 121, in resolve
    self._iremove(to_install, to_remove)
  File "/usr/local/psa/bin/yum_install", line 87, in _iremove
    self.install(name=p)
  File "/usr/lib/python2.6/site-packages/yum/__init__.py", line 3560, in install
    raise Errors.InstallError, _('No package(s) available to install')
InstallError: No package(s) available to install
FileFetcher: get file (~empty)/PSA_11.5.30/plesk-11.5.30-cos6-x86_64.inf3
FileFetcher: get file (~empty)/SITEBUILDER_11.5.10/sitebuilder-11.5.10-rhall-all.inf3
FileFetcher: get file (~empty)/BILLING_11.5.30/billing-11.5.30-rhall-all.inf3
FileFetcher: get file (~empty)/NGINX_1.5.0/nginx-1.5.0-cos6-x86_64.inf3
Error: The Yum utility failed to install the required packages.
Attention! Your software might be inoperable.
Please, contact product technical support.

Yum configuration file contains the exclude directives:

# grep exclude /etc/yum.conf
exclude=apache* httpd* mod_* mysql* MySQL* da_* *ftp* exim* sendmail* php* bind-chroot*

Cause

Yum is configured to prevent certain RPM packages from being considered by yum for installation or upgrade. This is done by enabling exclude directives in the /etc/yum.conf .

Resolution

Commented out the exclude directive string in the /etc/yum.conf :

~# grep exclude /etc/yum.conf
#exclude=apache* httpd* mod_* mysql* MySQL* da_* *ftp* exim* sendmail* php* bind-chroot*