Issue
On a newly created Related list, the "Edit" button is not visible, even after unchecking the "Omit edit button" checkbox.
In this case, even the "Enable Edit" button does enable the Edit button on the related list.
Release
All releases
Cause
On a newly created Related list, this will not work is because it is not a Related List but a "Relationship":
The record will have a name such as "REL:xxxxxx" where "xxxxx" is a sys_id of the related list. This determines that this is actually a Relationship, and relationships will not get the edit button when you click the Enable Edit button from List control.
To move records into and out of that related list you would update the target records so that they either matched the relationship criteria or make it so they no longer match.
Resolution
You can get the Edit button, not from the List Control, but directly editing the Edit Global UI Action.
"Edit" UI Action: nav_to.do?uri=sys_ui_action.do?sys_id=7fff4c3d0a0a0b3400ad3f1a1d613f74
You can either modify the condition field in the UI Action to fit your requirement or create a new global Edit UI Action.
*Keep in mind that any changes to this Edit UI Action will reflect throughout the instance since it is global.
Related Links
If the issue is not in a newly created related list, you should check if the user does not meet the criteria for the UI Action to be visible. This limitation can be due to an access control rule (ACL) that's failing or that the user does not meet the conditions defined in the UI Action record.
Review this article for more information: The New or Edit button does not appear on a one-to-many related list
