German and Namibian Inheritance Law differ fundamentally. The German Embassy Windhoek can support you when applying for a German certificate of inheritance or in renouncing an inheritance.
If you want to renounce an inheritance, you can declare the renouncement within a certain period of time.
Here you can find out what a certificate of inheritance is and how you can apply for it.
Information on the German inheritance tax is available (German language only) on the website of the German Federal Ministry of Finance.