Refer to GPT and yourself, this error is probably due to you using an undefined model in your analog circuit, including the "GND" model. This may be because you have not set up the emulation environment correctly or are using incorrect library files.
To solve this problem, follow these steps:
1 Confirm the simulation environment: Before simulating the circuit, you need to make sure that you have set up the simulation environment correctly. Make sure you have specified the correct supply voltage, circuit element parameters, connection mode, etc.
2 Check models: All components and models used in your circuit should be obtained from reliable sources and have been properly added to your LTspice library. If you have downloaded library files for components or models, make sure you have imported them properly into LTspice. If not, please import the file first.
3 Check component pins: Check that the component pins you use in the circuit are properly connected. Make sure that all pins are properly connected to other components or power supplies and that they match the schematic symbols.
4 Redefining GND: Try redefining GND if it is not correctly defined in your circuit. In LTspice, GND is defined as a special node and must be defined correctly. You can connect a ground node(for example, VSS) to the ground pins of all components in the circuit to define GND.
If the above steps do not solve the problem, please provide more details, such as the library file you are using, error information, so as to better help you solve the problem.