You need to understand that this is not running your software!
At this point, you are running the Keil Compiler, which is the first step in converting your human-readable source code into a machine-executable format.


The messages are there to enable you to identify the error, and fix it - so read them carefully!

eg,

From your 'C' (not C# - though it's probably the same) textbook, you should know that an "identifier" is the name of a variable, function, etc;
You should also know that an identifier must be defined before it can be used - and the compiler is telling you that this particular identifier has not been defined (ie, it is undefined)

So the solution to this error is to provide a definition for the identifier "TMOD"

Similarly for all the rest.

As Andy Curda said, these are probably supposed to be definitions for the 8051's SFRs (Special Function Registers), and these are normally supplied in a so-called "Header" File by the device manufacturer amd/or the compiler supplier...

Again, Header Files are a standard part of the 'C' language, so are covered in any 'C' textbook