I was also getting the same error. But now it is working for me.
First you install I2S microphone as shown here:  https://learn.adafruit.com/adafruit-i2s-mems-microphone-breakout/raspberry-pi-wiring-and-test

After that while you are installing I2S amplifier go to the detailed installation section and skip the first 2 steps and start from "[Create asound.conf file](https://learn.adafruit.com/adafruit-max98357-i2s-class-d-mono-amp/raspberry-pi-usage#create-asound-dot-conf-file-5-28)" and at last reboot your pi.
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