Hi Cynthia,
In-Kind donations of services are included on the 990 as Contributions on page 1, line 8, but are broken out in Section VIII, line 1g 'non cash contributions'. The non-profit must also list the donation as an expense which can, but does not have to, generate a 1099 depending on how the rules are set in the software.
Many donors will not provide an invoice for their services. These donors are not interested in the publicity of the donation and prefer not to have the extra accounting entries to list charitable contribution 'jobs'. It is not the non-profits responsibility to set a value. If no invoice is provided, no entries are made and the receipt can have no value.
Kirby Bowser