Can we use SQL Server express for Mac?

No, SQL Server will not run on macOS (it can run on Apple hardware, if you use Boot Camp and boot natively to Windows). Otherwise you will need to install virtualization software of some sort, where you install Windows in a VM, and install SQL Server there - I use Parallels Desktop, but there is also VMWare Fusion and Oracle VirtualBox.

You can also, of course, RDP to other machines, use SSMS equivalent clients to connect to another SQL Server running on Windows elsewhere, or install a different platform on the Mac (such as PostgreSQL). There is also an IDE from JetBrains called DataGrip and I'm sure others I don't know about.

With SQL Server 2017, you can run SQL Server on Linux, and can even host your own docker container without using Windows at all (I am doing this on my Macs right now - and I wrote about it here).

VS Code, Visual Studio, and Azure Data Studio now run natively on the Mac, so that can work for you too in some scenarios. It all depends on whether you expect SQL Server to actually run on macOS, or if you just want to do most of your work there.

Note: SQL Server database engine is available for Windows and Linux OS, but not for Mac. You can use a Windows VM or Docker Container to run SQL Server.