I created an Interface rule AS_portalReportColumn but later I realized that I ne

This happens on the old expression editor too for me. I think it's an error. I reported it internally to our folks who reported it to Appian.

Try after server restart once. This happens sometimes and will get resolved after server restart.

You can also try below steps:
1. Create a sample rule.
2. Create an application. Include only the above rule in application and export.
3. After export, edit the XML for the rule from zip and rename rule same as the one which you deleted.
4. Import the application again.
5. Check if rule is created.
6. Now again export the application with the same rule and rename the rule to something else(some dummy name) and import again.
7. Now try to create the rule with the name same as the one which is deleted.

Please let me know if it works.

Thanks Ashish.
I got your point to try to change the name of rule keeping the old id. Will give it a try.

This issue has been fixed with 7.9 Hotfix
AN-60172 - Low
It is now possible to create an interface, rule, or constant with the same name as a deleted interface, rule, or constant.