Library failed to install correctly

OS: Windows 10

Library: "Arduino AVR Boards", version 1.8.3

When installed this Library, then output in IDE gives the response:
"arduino:avr@1.8.3 installed"

But at the same time various data types are not recognized (e.g. String) : Unknown type name 'String'
Also pinMode parameters are not recognized (OUTPUT, INPUT) : Use of undeclared identifier 'OUTPUT'

In Arduino IDE log files is the error:

2021-07-13 22:53:08 daemon INFO Adding libraries dir [dir: C:\Users\xxxx\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.3\libraries, location: platform]
2021-07-13 22:53:10 root ERROR Uncaught Exception:
2021-07-13 22:53:10 root ERROR Error: Implementation error. Neither 'download' nor 'task' is available.
at ClientReadableStreamImpl. (C:\Users\xxxx\AppData\Local\Programs\Arduino IDE\resources\app\node_modules\arduino-ide-extension\lib\node\grpc-installable.js:16:23)
at ClientReadableStreamImpl.emit (events.js:223:5)
at addChunk (_stream_readable.js:309:12)
at readableAddChunk (_stream_readable.js:290:11)
at ClientReadableStreamImpl.Readable.push (_stream_readable.js:224:10)
at Object.onReceiveMessage (C:\Users\xxxx\AppData\Local\Programs\Arduino IDE\resources\app\node_modules@grpc\grpc-js\build\src\client.js:318:24)
at Object.onReceiveMessage (C:\Users\xxxx\AppData\Local\Programs\Arduino IDE\resources\app\node_modules@grpc\grpc-js\build\src\client-interceptors.js:291:178)
at C:\Users\xxxx\AppData\Local\Programs\Arduino IDE\resources\app\node_modules@grpc\grpc-js\build\src\call-stream.js:202:74
at processTicksAndRejections (internal/process/task_queues.js:76:11)
2021-07-13 22:53:11 root INFO <<< Boards package installation done.

What could be the solution of the problem?

Hi @fastis. Thanks for taking the time to give the beta phase Arduino IDE 2.x a try!

We are aware of these problems with the Arduino Language Server and have several reports in the bug tracker we are using to track the situation. Hopefully the reliability of this feature of the IDE will be improved over time.

For now, you'll need to just ignore these spurious results from the language server. As long as the sketch compiles correctly, you have nothing to worry about.

As you might expect from any beta software, there are some rough edges. If you want a stable experience from the IDE, please use Arduino IDE 1.8.15. The beta Arduino IDE 2.x is not appropriate for normal use at this stage in its development. But if you're interested in helping with the development work on the new Arduino IDE by testing and providing feedback, that is very much welcome. This is the reason we made it public in its current state: to offer the community the opportunity to participate in the work on what will become a cornerstone of the Arduino ecosystem in the years to come.

Thank you for the answer!

You're welcome.
Regards, Per