This is a common problem. SELLERS are often are very sensitive to unduly concerning employees to potential changes in ownership. In some cases, limited access, such as a walk-through, can be provided, but not access to knowledgeable on-site employees. The ASTM standard requires interviews with knowledgeable staff.
This should not prevent you from conducting your own independent evaluation of the property. Many useful resources can be accessed without involving on-site staff. An ESA report can be prepared using the available information, and the missing data can be referenced as an “exception” to the ASTM standard.