the issue here is that there are two types of wires that you are using. One is a standard simulink wire, which has the arrows indiciating flow of data. however, the other connections you are trying to use are actual electrical connections. These were designed so that you could build an actual circuit, that interacts with the more logical and math based simulink models. If you want to bring a signal from simulink into your circuit with a sum block, I would recommend using a controlled voltage source. https://www.mathworks.com/help/physmod/sps/powersys/ref/controlledvoltagesource.html
In the matlab control panel, you could type this: power_controlvolt
in order to open an example of how to use this type of block.