it is 20 degree today 这句英语有语法错误吗

语法没有错误。按照习惯应该写成Today is 20 degrees.或者Today's temperature is 20 degrees.

例句
1
Politicians have used television with varying degrees of success.
政客们已经利用电视取得了不同程度的成功。
2
We hire private security companies to run our prisons, send mercenaries to fight our wars, and pay for-profit schools to educate our children, all with varying degrees of success.
我们聘用私人安保公司来管理监狱，派遣雇佣兵去为国家打仗，出钱让盈利性学校来教育我们的孩子，这些都获得了不同程度的成功。
3
It's over 80 degrees outside.
外面的温度超过了80度。
4
Pure water sometimes does not freeze until it reaches minus 40 degrees Celsius.
纯净水有时候要降到零下40摄氏度才会结冰。
5
It was pointing outward at an angle of 45 degrees.
它向外指向45度角的方向。

The temperature today is 20 degrees Celsius
Today the temperature is 20 degrees Celsius
Today it is 20 degrees Celsius
Today it is 20 degrees Celsius outside