I'm not able to copy files from MacBook Air to Seagate hard disk

are you using any of seagate's preloaded software on the drive (or installed such software yourself) that supposedly makes the drive "Mac ready"? if yes, that could be the cause of your issue. when you use software from just about any manufacturer for their storage device you run the risk of that same software not being compatible with macOS updates / upgrades. it can cause the software to stop working. at best this will cause you a headache. at worst it can render your data inaccessible. it is always best to use disk utility to erase and format a drive before use to remove any preloaded software.

I strongly second Jeffery's comments. This has been my experience as well.