There are three stages to resolving this issue, which need to be done in order.
Check that the person approving has any approvers:
Log into FocalPoint admin
Click System control, Security Profile
On the appropriate profile, click Members
Find the person approving
Click Approvers
Select the appropriate module
Make sure there are users listed.
Check the listed approvers have access to the purchase request type and that they have appropriate sign off limit:
In FocalPoint admin
Click System option, Purchasing, Purchase request setup
On the purchase request type getting the above error, click Usage
Make sure the approvers are active and have a sign off limit higher than the purchase request.
Make sure approvers have permissions to approve purchase request:
In FocalPoint admin
Click System control, Security Profile
Edit the profile the approver is in
On the Security tab, click Authorisation
Make sure the Purchase Request check box is ticked.