really? I thought Doulingo would provide a Chinese keyboard on the screen.

You can still do the lessons by getting an English to Chinese translator on the web and then copy and paste the writing on to Doulingo. Of course, that would just be cheating, but considering your level I think it is appropriate enough for you to know this trick:)

https://www.writtenchinese.com/adding-a-chinese-keyboard-to-your-electronic-device/