You need to switch to parameterized queries for several reasons. Most importantly to avoid sql injection issues. Right now your code could easily be hacked and your database compromised. Secondly, you'll avoid the syntax issues.
Something like:
using (SqlConnection sqlCon = new SqlConnection("connection string"){
sqlCon.Open();
String sql = "select count(*) from zapasy_2020-21 where login= @username and odmiana= @odm";
using (SqlCommand cmd = new SqlCommand(sql, sqlCon){
cmd.Parameters.AddWithValue("@username", Context.User.Identity.Name);
cmd.Parameters.AddWithValue("@odm", odmiany.SelectedValue);
...
}
}