Found Solution!
We are using Dell Optiplex 3060. Which has a pre-installed operating system (Windows 10) with Dell Digital Library, Dell Encryption Management, and some other Dell apps.
Dell Encryption Management service causing the issue. It doesn't allow to install the script.
For those who are not using any of the Dell services can uninstall Dell Encryption Management service and Dell Digital Library service.
After deleting these apps we are now able to install 1809.
For those who are using Dell services, they have to approach Dell. It's a known issue for them and they are working on it.
Thank you all for your reply!