An error has occurred while executing Python code:

AttributeError: 'NoneType' object has no attribute 'GetGeoTransform'
Traceback (most recent call last):
  File "C:/Users/user/.qgis2/python/pluginsViewshedAnalysisviewshedanalysis.py", line 197, in run
    Algo)
  File "C:/Users/user/.qgis2/python/pluginsViewshedAnalysisdoViewshed.py", line 356, in Viewshed
    gt=gdal_raster.GetGeoTransform()#daje podatke o rasteru (metadata)
AttributeError: 'NoneType' object has no attribute 'GetGeoTransform'


Python version: 2.7.14 (v2.7.14:84471935ed, Sep 16 2017, 20:25:58) [MSC v.1500 64 bit (AMD64)]
QGIS version: 2.18.28 Las Palmas, aa30730ac6

Python Path:
C:/PROGRA~1/QGIS2~1.18/apps/qgis-ltr/./python/pluginsprocessing
C:/PROGRA~1/QGIS2~1.18/apps/qgis-ltr/./python
C:/Users/user/.qgis2/python
C:/Users/user/.qgis2/python/plugins
C:/PROGRA~1/QGIS2~1.18/apps/qgis-ltr/./python/plugins
C:Program FilesLaubwerkPython
C:PROGRA~1QGIS2~1.18binpython27.zip
C:PROGRA~1QGIS2~1.18appsPython27DLLs
C:PROGRA~1QGIS2~1.18appsPython27lib
C:PROGRA~1QGIS2~1.18appsPython27libplat-win
C:PROGRA~1QGIS2~1.18appsPython27liblib-tk
C:PROGRA~1QGIS2~1.18bin
C:PROGRA~1QGIS2~1.18appsPython27
C:PROGRA~1QGIS2~1.18appsPython27libsite-packages
C:PROGRA~1QGIS2~1.18appsPython27libsite-packagesjinja2-2.7.2-py2.7.egg
C:PROGRA~1QGIS2~1.18appsPython27libsite-packagesmarkupsafe-0.23-py2.7-win-amd64.egg
C:PROGRA~1QGIS2~1.18appsPython27libsite-packageswin32
C:PROGRA~1QGIS2~1.18appsPython27libsite-packageswin32lib
C:PROGRA~1QGIS2~1.18appsPython27libsite-packagesPythonwin
C:PROGRA~1QGIS2~1.18appsPython27libsite-packagesShapely-1.2.18-py2.7-win-amd64.egg
C:PROGRA~1QGIS2~1.18appsPython27libsite-packageswx-2.8-msw-unicode
C:PROGRA~1QGIS2~1.18appsPython27libsite-packagesxlrd-0.9.2-py2.7.egg
C:PROGRA~1QGIS2~1.18appsPython27libsite-packagesxlwt-0.7.5-py2.7.egg
C:/Users/user/.qgis2//python
C:/GIS