no idea where to start fixing this
to the right is a second landscape and the one that is all black both share the same material instance
If you use ready landscape material, probably auto material from Epic samples I guess, you should create layers required for material.
Go to landscape/paint and look for “Create Layer Info” (plus sign). Click + sign, save and set. Landscape should be no more black. Maybe it need additional layers, but you will see in layers list (can be few), without created Layer Info. All you need is to create all requires Layer Infos.