My wife did a contract job in December 2019 but paid in January 2020. Should this earing report in the 2019 Tax or 2020 tax?

She will report this in 2020 unless she operates a business that uses accrual-based accounting.  This is rarely used for individuals doing contract jobs - so it is not likely that it applies in her situation.  

Thanks for your answer. 

What I understand that her earing should be reported in 2020 tax (filling in 2021). Am I right? Please confirm. 

Yes, you are correct.  Your wife's earnings that she received in 2020 for work performed in 2019 will be reported on your 2020 tax return as she received the income in 2020.

Thank you very much for further clarification.

You are welcome!