This error is commonly caused by a user involved in the approval flow of the affected Purchase Request having been removed from their Security Profile.
Removing a user from their Security Profile actually causes all transactions they have been involved in to corrupt and error when you try and open them, so we instead advise that you create a profile with no permissions called something like "Leavers" or "Old users" and move the user into that, instead of deleting them. The user's account can still be disabled so that no one can log in as them and it does not use up a licence.
To resolve the error message, you just need to move the user back into a Security Profile again:
Log into the FinOps website
Click Purchasing, Purchase Requests.
Find and select the affected transaction.
Click History
Make a note of all users involved in the process.
Log into FinOps Administrator.
Click System Control, Users.
Check the list of users noted before against the user list here to find anyone that is missing.
Close the Users form.
Click System Control, Security Profiles.
Select the profile you want to move any missing users into.
Click Members.
In the list on the right hand side, find and select the missing user
Double click on them to move them into that profile.
Click OK to save. The error that occurred when opening the Purchase Request should now be resolved.