Airgradient DIY SoftwareSerial.h present but not found

I bought the Airgradient DIY as demonstrated in Your home's air could be making you sick. Fight back! - YouTube

It’s working at the moment. I changed some code. I now want to upload this code but I get the error

AirGradient.h:10:28: fatal error: SoftwareSerial.h: No such file or directory

I have no idea why this is happening because it used to work just fine before and I have SoftwareSerial.h located at /home/user/Downloads/arduino-1.8.19-linux64/arduino-1.8.19/hardware/arduino/avr/libraries/SoftwareSerial/src/SoftwareSerial.h.

I am using the Arduino IDE under Ubuntu Linux 22.04.

Are the search paths somehow not configured properly? How do I solve this?