playing the single player campaign and I'm not able to save, any solutions.

We're going to need a lot more information than that.

What are your system specs and Operating System? My first thought is that you are on Vista or later and UAC+DEP are protecting the installation folder preventing the save game file from being created, or creating it in your documents folder somewhere whilst the game continues to look for it in the game installation folder.

Also, which version of Earth 2140 are you using, the DOS or Windows version? GOG provide both as the DOS version has working AI whilst the Windows version, although looking better, has a crippled AI.

Might be worth sending in a support request to GOG too.

Try running the game as admin. It generally circumvents the whole UAC problem.