You Do Need power plug adapter while traveling from Guyana to Fiji.

Here is Why You Need An Adapter

The outlet and plug type used in Fiji is I and in Guyana are A, B, D and G. This is how they look like.

Guyana

In Guyana, outlets of type A, B, D and G are in use.

Type A

Type B

Type D

Type G

Fiji

In Fiji, outlets of type I are in use.

Type I