I'm unfamiliar with Google's (Vertex?) AI Platform and unable to test this hypothesis but it appears that the API uses location-specific endpoints.

Can you try configuring the client's ClientOption to specify the specific regional endpoint, i.e.:

Copyurl := fmt.Sprintf("https://%s-aiplatform.googleapis.com", location)
opts := []option.ClientOption{
    option.WithEndpoint(url),
}

And:

Copypackage main

import (
    "context"
    "fmt"
    "log"
    "os"

    aiplatform "cloud.google.com/go/aiplatform/apiv1"

    "google.golang.org/api/option"
    aiplatformpb "google.golang.org/genproto/googleapis/cloud/aiplatform/v1"
    "google.golang.org/protobuf/types/known/structpb"
)

const file = "MY_BASE64_IMAGE"

func main() {
    // Values from the environment
    project := os.Getenv("PROJECT")
    location := os.Getenv("LOCATION")
    endpoint := os.Getenv("ENDPOINT")

    ctx := context.Background()

    // Configure the client with a region-specific endpoint
    url := fmt.Sprintf("https://%s-aiplatform.googleapis.com", location)
    opts := []option.ClientOption{
        option.WithEndpoint(url),
    }

    c, err := aiplatform.NewPredictionClient(ctx, opts...)
    if err != nil {
        log.Fatal(err)
    }
    defer c.Close()

    parameters, err := structpb.NewValue(map[string]interface{}{
        "confidenceThreshold": 0.2,
        "maxPredictions":      5,
    })
    if err != nil {
        log.Fatal(err)
    }

    instance, err := structpb.NewValue(map[string]interface{}{
        "content": file,
    })
    if err != nil {
        log.Printf("QueryVertex structpb.NewValue instance - Err:%s", err)
    }

    rqst := &aiplatformpb.PredictRequest{
        Endpoint: fmt.Sprintf("projects/%s/locations/%s/endpoints/%s",
            project,
            location,
            endpoint,
        ),
        Instances: []*structpb.Value{
            instance,
        },
        Parameters: parameters,
    }

    resp, err := c.Predict(ctx, rqst)
    if err != nil {
        log.Fatal(err)
    }

    log.Printf("QueryVertex Res:%+v", resp)
}