Download and install the LiquidCrystal library.

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If you don't know how to install a library, visit these websites:

https://create.arduino.cc/projecthub/Arduino_Genuino/import-your-sketchbook-and-libraries-to-the-web-editor-296bb3

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https://www.arduino.cc/en/guide/libraries

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You can download the library from here:

https://www.arduinolibraries.info/libraries/liquid-crystal