Where is the custom_libs folder?
Viewing post in How to install OBS Notifier (4.9.1 and 4.9.1-compat) comments
As mentioned in the same 3rd point, you need to download 1.0.5 or lower. https://github.com/DmitriySalnikov/OBSNotifier/releases/tag/1.0.5
But also, as it says in the title, this is only necessary if you are using an older version of OBS and/or OBS Notifier.