Hello,
the main issue is the parameterization of the dc-dc power supply. You can see by the error messages what the specific issues are. You can try increasing the load side current. You can also add a capacitor to the output and input of the dc/dc converter, and initialize those capacitors to the correct voltages. This is on the variables tab.
In general, it is quite challenging to combine specialized power systems and simscape models, especially if you are new to the tools. I would recommend using one or the other. If you want a simscape fuel cell, look here: https://www.mathworks.com/help/physmod/sps/ref/fuelcell.html?searchHighlight=fuel%20cell&s_tid=srchtitle.
Otherwise, It can be good to try and experiment with the different connection options. You can also try adding snubber resistors to both sides of the connection points between the two models to see if that helps (large resistors that take 1% of the expected load).
Another option is that you can build the whole system in specialized power systems.
Regards,
Joel
