尝试

 comboBox2.DataSource = ds.Tables [0] ; 

注意：

格式化sql查询字符串是易受攻击到 SQL注入 [ ^ ]攻击

总是使用参数化查询以防止SQL Server中的SQL注入攻击 [ ^ ]

不是你问题的解决方案，而是你遇到的另一个问题。

永远不要通过连接字符串来构建SQL查询。迟早，您将使用用户输入来执行此操作，这会打开一个名为SQL注入的漏洞，这对您的数据库很容易并且容易出错。

名称中的单引号你的程序崩溃。如果用户输入像Brian O'Conner这样的名称可能会使您的应用程序崩溃，那么这是一个SQL注入漏洞，崩溃是最少的问题，恶意用户输入，并且它被提升为具有所有凭据的SQL命令。

SQL注入 - *** [ ^ ]

SQL注入 [ ^ ]

Hello Guys
can you help me with this

What I have tried:

private void button1_Click(object sender, EventArgs e)
      {
          {
              comboBox2.Enabled = true;
              string klijent = comboBox1.Text;
              CON.Open();
              string sqlquery = "SELECT Naziv FROM [Dobavljaci_" + klijent + "] WHERE NazivFirme = '" + klijent + "'";
              SqlCommand cmd = new SqlCommand(sqlquery, CON);
              SqlDataAdapter da = new SqlDataAdapter(sqlquery, CON);
              DataSet ds = new DataSet();
              da.Fill(ds);
              cmd.ExecuteNonQuery();
              comboBox2.DisplayMember = "Naziv";
              comboBox2.ValueMember = "Naziv";
              comboBox2.DataSource = ds;
          }
          {
              string klijent = comboBox1.Text;
              string sqlquery1 = "SELECT stopa FROM NKlijent WHERE NazivFirme='" + klijent + "'";
              CMD = new SqlCommand(sqlquery1, CON);
              SqlDataReader sdr1 = CMD.ExecuteReader();
              while (sdr1.Read())
              {
                  label1.Text = sdr1["stopa"].ToString();
              }
              CON.Close();
          }

On Label7 i am getting value from data base but on combobox I am getting this

System.Data.DataViewManagerListItemTypeDescriptor

try

comboBox2.DataSource = ds.Tables[0];

Note:
Formatting the sql Query string is vulnerable to SQL Injection[^] attacks
always use Parameterized queries to prevent SQL Injection Attacks in SQL Server[^]

Not a solution to your question, but another problem you have.
Never build an SQL query by concatenating strings. Sooner or later, you will do it with user inputs, and this opens door to a vulnerability named "SQL injection", it is dangerous for your database and error prone.
A single quote in a name and your program crash. If a user input a name like "Brian O'Conner" can crash your app, it is an SQL injection vulnerability, and the crash is the least of the problems, a malicious user input and it is promoted to SQL commands with all credentials.
SQL injection - Wikipedia[^]
SQL Injection[^]