State Machine for SPI Master/Slave Interface

di_i is a std_logic_vector, so to assign it to the data in variable you need to type convert it:

data_in <= to_integer( unsigned( di_i) );

And you still have the same problem that you're doing an unnecessary (and illegal) type conversion for the data_out assignment from the data variable.

There are other type conversion problems. I suggest fix the syntax errors first before worry about the functionality.