The OptiPlex 780 was released in Late 2009. Most would have been sold in 2009-2011 and with 4 years warranty standard, pretty much all units have expired warranty. Dell therefore are extremely unlikely to allocate resources to testing this model.
However the OptiPlex 780 is a robust Intel machine. Pretty much all the system drivers will be inbuilt into Windows 10 TH2 for it.
Make sure all systems are on Legacy BIOS revision A15:
This system doesn't have UEFI and SecureBoot.
Then made a Dell Backup and Recovery 16 GB USB flash Drive (using the latest version):
Then you can proceed with the clean install (recommended) or upgrade install (not recommended):
When making a Bootable USB use the MBR partition scheme for legacy BIOS.
Some additional tutorial videos are here:
Others have reported successful installs: