Prueba a cambiar esa línea así:
C_COMP_COMMAND=%C_INSTALL%\bin\bcc64.Exe -c -I%HB_INSTALL%\include -I%C_INC_INSTALL% -I%C_INC_INSTALL%\windows\sdk -I%C_INSTALL%\include\windows\crtl -o%OBJ% %C%
x:\harb\harb2020\bin\hbmk2 -obin\arbol.exe arbol.hbp -comp=bcc