Hi Didem,
It is my understanding that, you can see red dashed lines while connecting the blocks in Simulink.
The red dash mean you cannot connect the two blocks together. I assume that would be due to the wrong connections between blocks made or extra connections done, or it might be connected to only one block.
These red dashed lines can be removed by clicking on them and using the delete button on the keyboard and making correct connections.
You may refer to the following links for more information:
