msgraph-sdk-go can't create more than two extensions on a user

When I try to create an schema extension on a user like this

schemaExtension := graphmodels.NewSchemaExtension()
additionalData := map[string]interface{}{
	"extensionName": "dean.ext.test.1",
	"theme":         "dark",
	"color":         "purple",
	"lang":          "English",
}
schemaExtension.SetAdditionalData(additionalData)

if result, err := client.UsersById(userId).Extensions().Post(context.Background(), schemaExtension, nil); err != nil {

I get this error:

Error: error status code received from the API
    code: BadRequest
    msg: Maximum number of extensions values supported per application is 2.

But I have not created any schema extensions on this user. I created two open extensions, but I should be able to create additional schema extensions.

Why does the error message say that extensions are per application? The code above is trying to create an extension on a particular user, not an application.

I want to remove the extensions on this user, but there is nothing I can find in the portal that shows extensions for a user. Where can I find the extensions on a user in the portal?

The portal does show user attributes which seem to apply to all users. Are user attributes related to extensions? How can I access these user attributes using the msgraph-sdk-go?

The msgraph-sdk-go is currently at v 0.55 and is a non-production preview.
After discussing this with some colleagues we've decided to drop the MS Graph SDK and use the v 1.0 Graph REST endpoints directly. They've had success with that approach and have found that the SDK doesn't help very much.

I tried to reproduce the same in my environment and got below results:

When I ran below request to create extension via Graph Explorer, I got same error as you like below:

POST https://graph.microsoft.com/v1.0/users/<userID>/extensions

{
    "extensionName": "dean.ext.test.1",
    "theme": "dark",
    "color": "purple",
    "lang": "English"
}

Response:

> Note that, you can create only 2 open extensions for any directory
> resource like user, group, device etc... To confirm that, you can
> check this MS Doc on limits.

The error usually occurs if you already have 2 open extensions existing in your directory. You can make use of below MS Graph query to list open extensions on a user.

GET https://graph.microsoft.com/v1.0/users/<userID>/extensions

Response:

You can get the same results using msgraph-sdk-go with below code:

graphClient := msgraphsdk.NewGraphServiceClient(requestAdapter)

result, err := graphClient.UsersById("user-id").Extensions().Get(context.Background(), nil)

To delete extension from user, you can use below query by including extension name like this:

DELETE https://graph.microsoft.com/v1.0/users/<userID>/extensions/<extension_name>

Response:

You can delete open extension from user using msgraph-sdk-go with below code:

graphClient := msgraphsdk.NewGraphServiceClient(requestAdapter)
  
graphClient.UsersById("user-id").ExtensionsById("extension-id").Delete(context.Background(), nil)

When I ran the query to list open extensions now, I got only 1 extension in response as other one is deleted successfully like below:

GET https://graph.microsoft.com/v1.0/users/<userID>/extensions

Response:

The code you are currently using to create schema extensions is creating open extensions.

To create schema extension using msgraph-sdk-go, once cross-check your code with below sample code:

graphClient := msgraphsdk.NewGraphServiceClient(requestAdapter)

requestBody := graphmodels.NewSchemaExtension()
id := "domainName_schemaName"
requestBody.SetId(&id) 
description := "My schema extension"
requestBody.SetDescription(&description) 
targetTypes := []string {
	"User",

}
requestBody.SetTargetTypes(targetTypes)


extensionSchemaProperty := graphmodels.NewExtensionSchemaProperty()
name := "theme"
extensionSchemaProperty.SetName(&name) 
type := "String"
extensionSchemaProperty.SetType(&type) 
extensionSchemaProperty1 := graphmodels.NewExtensionSchemaProperty()
name := "color"
extensionSchemaProperty1.SetName(&name) 
type := "String"
extensionSchemaProperty1.SetType(&type) 
extensionSchemaProperty2 := graphmodels.NewExtensionSchemaProperty()
name := "lang"
extensionSchemaProperty2.SetName(&name) 
type := "String"
extensionSchemaProperty2.SetType(&type) 

properties := []graphmodels.ExtensionSchemaPropertyable {
	extensionSchemaProperty,
	extensionSchemaProperty1,
	extensionSchemaProperty2,

}
requestBody.SetProperties(properties)

result, err := graphClient.SchemaExtensions().Post(context.Background(), requestBody, nil)

Reference: Create schemaExtension - Microsoft Graph v1.0

UPDATE:

As mentioned in your answer, THE GO SDK IS IN PREVIEW. IT IS FOR NON-PRODUCTION USE ONLY. So, it's better to stick with REST API calls, for now, to manage resources via MS Graph.