Hi @Shaney,
Currently, Flow does not support people outside the organization for approval. Please ensure that user is a member of the organization.
If the user is not a member of the organization but needs him to participate in the approval process, you could try to use Send email with option.
In addition to management access rights, you could try to select a document, and then click Manage Access, you could add user, and give the user the corresponding permissions, provided that you are owner.

Best Regards,
Barry