This error is caused by a custom field of type dropdown on the requisition header. Let say there are there option to select :
A new requisition REQ1 is created and on this custom field the value 'User 1' is selected. REQ1 is ordered.
Then the custom field is updated so that one of the option is removed, let say User1 is removed. So the options available are now :
Now when we copy the requsition REQ1 and try to submit, we will see an error saying 'Please fix the errors below' but we will not see where the error is.
The above error occurs because we copied the data from REQ1 to REQ2, where the custom field value 'User1' has been deleted.
In scenario when this field is restricted by content group and if the user who copies this requisition does not have the necessary content group assigned,
thus cannot see this custom field nor is able to change the value. Although the custom field is not a required fields for this user, the requisition still has an
invalid data 'User1' so the system give an error when copying and submitting the requisition.
If we create a new requisition, the error should not occur. Another solution would be to assign the user to the content group that the custom field is assigned to,
or remove content group restriction from the custom to field.