This article provides a solution when users are unable to change the parameter selected for finding Sales Documents on the Advanced Find tab.
The above issue was determined to be a defect (EVCID-18416) which Engineering fixed in v8.0.27.
The recommended solution for this issue is to upgrade to this version or newer.
For more information, see the attached Release Notes where this issue is registered as EVCID-18416 - Find Documents using Document number.
After upgrading Everest, the search parameter should be able to be modified for better filtering by the selected parameter.