The plugin installation information on this page,

https://www.pulumi.com/registry/packages/equinix/installation-configuration/#provider-binary

does not include the --server parameter.
The server parameter is needed when manually installing the Equinix plugin since it is not hosted by Pulumi and so throws a 403 error when installed as documented.

Instead, the docs should reflect the need to specify the server as in:

pulumi plugin install resource equinix v0.2.1 --server github://api.github.com/equinix