You Do Need power plug adapter while traveling from Serbia to Namibia.

Here is Why You Need An Adapter

The outlet and plug type used in Namibia are D and M and in Serbia are C and F. This is how they look like.

Serbia

In Serbia, outlets of type C and F are in use.

Type C

Type F

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M