This problem was reported on Release Automation 6.5.
However, it is possible to occur on all version.
Attributes in action settings refered to some parameters. One of parameters was not defined properly.
As a result, RA could not replace the parameter to actual value.
In this case, deleted all references to the parameter from action.
And then, the process could be published.
As another solution, you can define necessary parameters properly as well.