Unable to use .class.st files

[Zokalyx]

I'm trying to use .class.st files but they don't seem to do anything. Formatting on those files fails:

[Error - 12:29:09 PM] Request textDocument/formatting failed.
  Message: Internal JSON-RPC error.
  Code: -32603 
[object Object]

I'm using the default Pharo version downloaded by the extension. Windows 10.

This is my test file:

Class {
    #name: #MyClassTest,
    #superclass: #TestCase,
    #instVars: [ ],
    #category : #'MyPackage'
}

{ #category : #accessing}
MyClassTest >> testMethod1 [
    ^ 'hello'
]

{ #category : #accessing}
MyClassTest >> testMethodName [
    self fail
]

I'm new to Pharo so there might be something obvious I'm missing!

[Zokalyx]

Image: Moose64-12-PLS Server version: v5.0.0 VM version: 120

Edit: Even though I downloaded the latest language server, the command "Show language server version" shows 1.0.0.

[Zokalyx]

I have used the "Pharo: Create new project" command and this seems to have done something. However, all actions on class.st files still fail.