You will get this error message if the object does not have enough UV channels to render the material applied to it. In Maxwell Render, UV channels (UV sets) start from 0. This means that, if an object only has one set of UVs, but you apply a material which has a texture that uses UV channel 1, the object needs to have two sets of UV channels (0 and 1).
If your object only has one UV channel, make sure all textures (diffuse, bump, weightmap etc.) are set to use the projector channel 0 in the Texture Picker.
If the material requires two or or more UV channels, add these extra UV channels to the object the material is applied to in your main application, or in Maxwell Studio from the UV sets section of the Object parameters panel. For more details see the Working with UV sets and textures page in the Maxwell Documentation section.