What is this ß thing?
"ß" is not a "B" or "β" (beta)!
"ß" is used after long vowels and diphthongs (ai/eu/au/äu), and "ss" is used after short vowels.
How to type ä, ö, ü, and ß
On the website version of Duolingo, you can use the on-screen buttons below the box where you're supposed to write your answer. Use the arrow to toggle between lowercase and uppercase letters.
In order to type "ä" on a touch device, press and hold "a" and select "ä", to type "ö", press and hold "o" and select "ö", to type "ü", press and hold "u" and select "ü", to type the "ß" character on a touch device, press and hold the "s" key and select "ß".
If this doesn't work, go to your keyboard settings on your device and enable the German keyboard.
In modern German, you can easily write ss, ae, oe, ue in exchange for ß, ä, ö, ü.
So I wrote "his name is Karl", same same but more accurate phraseology in English. I find Duolingo annoyingly obtuse!
The possessive determiners related to personal pronouns do not have apostrophes in English.
We do not write "m'y name, you're name, he's name, she's name, it's name, we's name, you're name, they're name".
The correct spellings are "my name, your name, his name, her name, its name, our name, your name, their name".