How some of the people were using "kotlin" although it is not in the sololearning list.

Answer 1

Kotlin playground is now available only for beta users....
https://www.sololearn.com/Discuss/1048370/?ref=app
https://www.sololearn.com/discuss/1061478/?ref=app
https://www.sololearn.com/discuss/1054493/?ref=app

Answer 2

//available 4 beta version currently
https://www.sololearn.com/Discuss/1063066/?ref=app
https://www.sololearn.com/Discuss/1057552/?ref=app
https://www.sololearn.com/discuss/1052386/?ref=app

Answer 3

but why not us or other can use it, whats the reason.

Answer 4

Good news!
Kotlin is now live in SoloLearn!

https://www.sololearn.com/Discuss/1319472/?ref=app
https://www.sololearn.com/learn/7792/?ref=app