I upgrade my system to macOS big sur but I can not install Itunes.

Big Sur includes Music, TV, Podcasts- applications that have subsumed some of the roles that iTunes used to perform.

Some others are now done by the Finder.

It is quite a lot better this way. iTunes was extremely bloated.

You can still access your old iTunes library using Music.

Apple split iTunes up in Catalina. Use the new applications instead.

(196527)