database is getting locked on insert statements

I have a problem when I try to insert into my sqlite db. I have two tables looking like this :

CREATE TABLE user(UId INTEGER PRIMARY KEY ASC AUTOINCREMENT ,name VARCHAR(100) UNIQUE,phoneNumber INTEGER UNIQUE);

and

CREATE TABLE 'transaction'(TId INTEGER PRIMARY KEY ASC AUTOINCREMENT , loan REAL, trackUser INTEGER,FOREIGN KEY(trackUser) REFERENCES user(UId));

I can insert into the user table without any problems like this:

INSERT INTO user (name, phoneNumber) VALUES(@name,@phoneNumber)

But when I try to add a transaction my database gets locked. I do that like this:

    public void Add(Transaction trans,User user)
    {
        _conn.Open();
        string SQL = "INSERT INTO 'transaction' (loan, trackUser) VALUES(@loan,@userID);";
        string checkUserSQL = "SELECT EXISTS(SELECT 1 FROM user WHERE UId = @UId) LIMIT 1;";
        SQLiteCommand checkUserCommand = new SQLiteCommand(checkUserSQL,_conn);
        checkUserCommand.Parameters.Add(new SQLiteParameter("@UId", user.UId));
        var reader = checkUserCommand.ExecuteReader();
        reader.Read();
        var check = reader.GetBoolean(0);
        if (!check)
        {
            throw new UserDoesNotExcistException();
        }
        SQLiteCommand transCommand = new SQLiteCommand(SQL, _conn);
        transCommand.Parameters.Add(new SQLiteParameter("@loan",trans.Loan));
        transCommand.Parameters.Add(new SQLiteParameter("@userID",user.UId));
        transCommand.ExecuteNonQuery();
        _conn.Close();
    }

the select statement that checks if the user exicst runs without a problem but when it comes to transCommand.ExecuteNonQuery(); the db gets locked. Am I binding the forign key in a wrong way or why is my db getting locked?