It's complaining about unable to solve for initial conditions. There may be conflicting or inconsistent initial variable values set in some blocks without you realizing that. It looks like you are working with Gas domain, so one thing to look out for is avoid pushing a large flow rate at time 0. Other blocks in your system, pipes, volumes, etc., may not have the I.C. set up to be ready to push the specified flow rate at your source. Sometimes ramping up the flow rate from zero to target value helps.
If you could share the model, I can take a look.
