The openwebif action is not available for installing through GUI (openhab 2.5M2).

https://github.com/openhab/openhab1-addons/tree/master/bundles/action/org.openhab.action.openwebif/src/main/java/org/openhab/action/openwebif/internal

Issue seems to exist for a a longer period of time according to this community post:
https://community.openhab.org/t/where-to-find-openwebif-action-oh2-4/60630

Manually installing the jar in addons folder works fine.