Digital Elevation Model (DEM) exported from Agisoft Metashape Professional uses floating point TIFFs. Most standard image viewers and editors can not recognize them. We recommend to use dedicated GIS applications to work with DEM, for example, Global Mapper or Quantum-GIS.
If you would like to save only visual representation of the DEM in false colors (RGB), like it is displayed in Ortho view window of Metashape Professional, then use Palette option in Raster Transform section of Export DEM dialog.