I don’t think this expression returns a boolean. What kind of custom field is that? And what were you trying to accomplish with the “not” operator?
In the expression that does work, you have an ==. If you were just trying to make sure it has a value, then maybe you want issue.customfield_10077 !== null.
I had also tried issue.customfield_10077 != null, which gives me a “An error occurred during installation. Contact the app vendor for support.” error
The reason I’m using the ! operator is by a suggestion from @DarrylLee in this community post. In there I have a screen-shot where the not operator returns a false (boolean) value when tested with the Expression Tester app
Atlassian support provided me with the logs of my app installation. It was not very clear what the problem was, but entries made reference to uninstalling the app because of an issue in another module I had in the same descriptor, a module that we’ve had deployed for months. After separating jiraWorkflowValidators into another descriptor if worked well with expression