Error code:

type MongoConn struct {
	clientOptions *options.ClientOptions
	client        *mongo.Client
	collections   *mongo.Collection
}

var mongoConn *MongoConn

func InitMongoConn() error{

	ctx, cancelFunc := context.WithTimeout(context.Background(), 10*time.Second)
	defer cancelFunc()

	mongoUrl := "mongodb://" + user + ":" + password + "@" + url + "/" + dbname
	mongoConn.clientOptions = options.Client().ApplyURI(mongoUrl)
	
	//......
}

To solve the problem caused by pointer assignment:

var mongoConn MongoConn