Babylon is using one UBO (Uniform Buffer Object) per light to pass light information to the shader.
On my GTX 1080 I have 24 UBOs available in WebGL2. It seems on your card the limit is 16. Note that Babylon is also using one UBO for scene data and one UBO for the material data, so in the end you would need 18 UBOs (hence the 18 in the error message).
Note that 18 lights is a very high number, you normally don’t want so many lights active for a mesh!