See https://www.mathworks.com/help/simulink/gui/simulink-preferences-model-file-pane.html for the "Do not load models created with a newer version of Simulink" preference and turn it off.
The loaded model might not be completely compatible with your older version. You would be better off getting access to a newer version of Simulink and using it to export a file compatible with your release.
Note: The file was saved in R2020b. If it had been saved even one release later, in R2021a, then the model would have been incompatible with older releases. The libraries were reorganized in R2021a. But fortunately for you, in R2020b that change had not been done yet.
