This seems similar to the question in this post: https://community.pix4d.com/t/3702
External point clouds cannot be imported to generate the 3D Textured Mesh. External Point Clouds can only be used to create outputs such as the Digital Surface Model (DSM) and Orthomosaic that are created in step 3. DSM, Orthomosaic and Index. There is an article that summarizes this topic : https://support.pix4d.com/hc/en-us/articles/202560049