How to add rank in ranked list in RTC Plans?

As Ralph mentioned, the article: https://jazz.net/library/article/1364 will help to show you how to rank your work items.
The priority attribute is a drop down - rank is a little bit different. I would advise you to review the above link for more information on how to configure it.
If you are trying to change the rank number, or use the apply rank action - and it is not saving or working as expected, I would encourage you to open a PMR with IBM support so they can take a look at your use case and the behaviour and recommend how to resolve it.

 
Ranking work items
You can rank work items to prioritize them according to your requirement or expected business value. By default, the work item's Priority attribute is used as the planning Ranking attribute. A change to a work item's priority changes its overall ranking. As a result, changing an item from Medium to High priority moves it up in the overall ranking, while changing it from High to Low moves it down. For work items with the same priority, you can set their relative ranking order by dragging and dropping, which is a way of manually sorting them within priority. If you drop an item into a different priority grouping, its priority automatically changes to match the items in the group where it is dropped. The work item is ranked after the item it is dropped under.
Setting the ranking attribute
About this task
You can set the ranking attribute in the Plan Attributes editor.
Procedure
In the Team Artifacts view, right-click the project area to customize, and click Open. Click the Process Configuration tab.
Click Project Configuration > Configuration Data > Planning, and then click Plan Attributes.
In the Ranking Attribute section, select the Attribute type.
Note: You can add custom attributes to the Ranking Attribute section by expanding the Attribute Mappingsection, and clicking Add. In the Attribute Mapping window, select the attribute to add, and click OK.
Ranking work items
About this task
You can rank work items by dragging and dropping them between work items in the Planned Items tab.
Procedure
Click a work item and drag it. As you drag the work item, a black line indicates where the work item can be inserted.
Release the mouse pointer to insert the work item at the point of the black line.