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

Here is Why You Need An Adapter

The outlet and plug type used in Namibia are D and M and in Sri Lanka is G. This is how they look like.

Sri Lanka

In Sri Lanka, outlets of type G are in use.

Type G

Namibia

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

Type D

Type M