This error is caused by the user not having enough permissions in Dimensions to create Purchase Orders. When a Purchase Request in FocalPoint is fully approved and posted, it then creates the corresponding order in Dimensions and so users need the appropriate permissions in there too.
To assign the required permissions to the profile the user is in:
Log into Dimensions
Browse to System, Profiles
Scroll through to the profile that the affected user is in
Click Edit
Click on the red cross beside "Purchase Orders" to change it to a green tick (if it is already a green tick, continue to the next step)
Click on the words "Purchase Orders" to expand the section
You can disable all sections other than "Transactions" as required, by clicking on the green tick to turn it to a red cross. You may also be able to disable some options within Transactions, but you will need to do this on a trial and error basis as some options will be required depending on your Purchase Request setup
Click Save to save your changes
Alternatively, you can move the user into a profile which already has sufficient permissions if you do not want to make changes that affect all users in this profile.