Deepa, it is likely a 'path not found' issue. Add as many paths as relevant to help the IDE find selenium and webdriver. For example, on my Mac, I've added all the following paths so nothing is missed :
file:///Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5 file:///Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/plat-darwin file:///Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/lib-dynload file:///Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages file:///Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/selenium-3.0.0b2/py file:///Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/selenium-3.0.0b2/py/selenium file:///Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/selenium-3.0.0b2/py/selenium/common file:///Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/selenium-3.0.0b2/py/selenium/webdriver