Newtonsoft.Json unfortunately seems to be incompatible with this version of Unity, so either download and use an older version or consider reaching out to Unity for support.
Unfortunatly i started to program the whole game in this version, so i don´t really wanna copy and paste and do that stuff to another version. But thank you for help and i will try to get reply from Unity support.
I was having the same problem with this Unity version but managed to download it by clicking on "Add package by name..." instead and pasting: com.unity.nuget.newtonsoft-json
