It comes down to the number of samplers. There’s limits on how long a program a GPU can run on a given pixel, how many textures you are sampling, etc.
I grabbed this once upon a time, but you should double check: A sampler is something that tells the graphics card how to sample the textures (linear filtering, point filtering, wrap addressing, etc). The 16 sampler limit is a d3d11 limit and not something we can change. HOWEVER you can use fewer samplers in your material with the shared sampler setting to use up to 128 unique textures.
W/o looking at the material I couldn’t say for sure but you are likely running into some kind of limit on the landscape material.
On the material, what’s the material stats, the little window at the bottom say? It ought to look like this?
Base pass shader: 533 instructions
Base pass shader with Surface Lightmap: 517 instructions
Base pass shader with Volumetric Lightmap: 533 instructions
Base pass vertex shader: 58 instructions
Texture samplers: 3/16
Texture Lookups (Est.): VS(3), PS(46)
Virtual Texture Lookups (Est.): 15
Virtual Texture Stacks: 2
User interpolators: 16/16 Scalars (4/4 Vectors) (TexCoords: 8, Custom: 8)
You also have one in your material-instance window, what is that one?
Are you using a market-product or a thing from te web?