How to set available actions based on an attribute value in RTC Workitem Workflow?

You can not do that in RTC. This has been answered before and RTC for all I know does not provide means to make the workflow dynamic like that. RTC was designed as a lightweight work item management tool and not something that can be customized to that extend. A work item type (category) has one workflow and all of the items have the same workflow.

You can create an advisor to prevent a state change based on an attribute value or more complex rules, but you can not suppress the available actions. You can only fail the save. See https://rsjazz.wordpress.com/2014/05/26/only-owner-can-close-workitem-advisor/ as an example.

also, please follow the slides here https://rsjazz.wordpress.com/2016/07/15/rtc-process-customization-what-you-can-and-cannot-do/ carefully for all the other limitations that frequently come up.