Hi Sherif,
Regarding your error message:
Unexpected or invalid block connection. This can arise when a library block changes its underlying implementation to use a new domain that is incompatible with the original implementation. Check and reconfigure block connections to use a consistent domain.
It is hard to say the exact cause of the issue without seeing the model, but it is likely that the blocks in your model belong to different domains.
Simulink has several different physical modeling toolboxes that represent different domains. Each domain has different physical connections associated to it and with the exception of a few multi-domain blocks, no two domains are allowed to connect together. SimElectronics and SimPowerSystems are an example of two different domains which do not connect together but can be used to model similar situations.
In your screenshot above, the Potentiometer block is from the SimElectronics library while the VM (Voltage Measurement), Gnd (ground) and powergui block are from the SimPowerSystems library. If you would like to measure the voltage across the Potentiometer I suggest replacing the VM and Gnd blocks with block from the SimElectronics library (such as the Voltage Sensor and Electrical Reference blocks).
~Zack
