Compiler error when using ArraySegment

I'm using VisualStudio 2022 community & unity 2020.3.33f1.

I wrote some code in a VS project without errors & coppied the them into unity folder and open them. Then:

• it causes an error : ArraySegment doesn't contain Slice().

• another error caused by`MyArrSegment[i]`, "[i]" causes error.

• byte[]can't automaticly cast to ArraySegment<byte>.

Is the language version too low? Can it be solved? Please help.