Here are some errors you might encounter:

Voltage source loop with no resistance! - this means one of the voltage sources in your circuit is shorted. Make sure there is some resistance across every voltage source.

Capacitor loop with no resistance! - it's not allowed to have any current loops containing capacitors but no resistance. For example, capacitors connected in parallel are not allowed; you must put a resistor in series with them. Shorted capacitors are allowed.

Singular matrix! - this means that your circuit is inconsistent (two different voltage sources connected to each other), or that the voltage at some point is undefined. It might mean that some component's terminals are unconnected; for example, if you create an op-amp but haven't connected anything to it yet, you will get this error.

Convergence failed! - this means the simulator can't figure out what the state of the circuit should be. Just click Reset and hopefully that should fix it. Your circuit might be too complicated, but this happens sometimes even with the examples.

Transmission line delay too large! - the transmission line delay is too large compared to the timestep of the simulator, so too much memory would be required. Make the delay smaller.

Need to ground transmission line! - the bottom two wires of a transmission line must always be grounded in this simulator.