How to generate product id that contains letter and numbers in C#

I personally would not generate the ID in the C# code I would use the MySQL database to generate the id for me: See MySQL :: MySQL 5.7 Reference Manual :: 3.6.9 Using AUTO_INCREMENT[^]

I certainly would never use a method that relies on me incrementing the "previous" number as it will not work in a multi-user environment.

Once you have that numeric Id you can display it however you want e.g. (Not tested!)

String.Format("PC{0}//POS", id.ToString(D2));

although I would also not limit my output to 1 to 99 .. use D5 at least.

If you really, really want to store the text version on the database then you could use a trigger (MySQL: AFTER INSERT Trigger[^]) to capture the id and generate the text version

[EDIT after OP comment]
Try something like the following (note again, untested!)

CREATE TRIGGER product_id
 AFTER INSERT
 ON test FOR EACH ROW
 	UPDATE test set textid = concat('PC', LPAD(NEW.id, 5, '0'), '/POS') WHERE id=NEW.id;

Explanation:
The trigger is called product_id and it fires after a row is, or rows are, inserted into the table test
There is only one statement in the trigger so I haven't used BEGIN ... END, but you have already demonstrated that you know how to do that with your code in the comments - I'm just being lazy (and demonstrating a point). (Note, if you do have more than one statement you will need to change the delimiter when creating your trigger - see the documentation)
The trigger is going to UPDATE the table test using NEW.id. The special table NEW contains ... the new rows. You can't update it. There is another special table OLD which contains the old values too.

I'm sorry I haven't been able to test this properly and this isn't the correct place to tutor you further, but at least now you have some search terms to help your further research.

[EDIT - further code samples after OP comments. This solution is hidden in the reams of comments below].
Both of these will work

DELIMITER //

CREATE TRIGGER product_id
 AFTER INSERT
 ON test 
 BEGIN
      String.Format("PC{0}//POS", id.ToString(D2));
 END; //
DELIMITER ;

or

CREATE TRIGGER product_id
 AFTER INSERT
 ON test 
      String.Format("PC{0}//POS", id.ToString(D2));

Specifically, if you want to include a BEGIN and END you MUST use DELIMITER as well!

UPDATE table set textid = concat('PC', LPAD(autoid, 5, '0'), '/POS');

 One or more errors have occurred while processing your request:
The following query has failed: "CREATE DEFINER=`root`@`localhost` TRIGGER `product_code` AFTER INSERT ON `inventory_table` FOR EACH ROW CREATE TRIGGER product_code AFTER INSERT ON test FOR EACH ROW BEGIN UPDATE test set textid = concat('PC', LPAD(NEW.id, 5, '0'), '/POS') WHERE id=NEW.id; END"

MySQL said: #1303 - Can't create a TRIGGER from within another stored routine 

DELIMITER //

CREATE TRIGGER product_id
 AFTER INSERT
 ON test 
 BEGIN
      String.Format("PC{0}//POS", id.ToString(D2));
 END; //
DELIMITER ;

That's a whacky question, but here ya go:

public static string IncrementID2(string startValue, int numNonDigits)
{
    int max = 999999999;
    string[] parts = startValue.Split('/');
    // is it safe to assume that the third splitted string is the number you're looking for?
    int number;
    if (Int32.TryParse(parts[2], out number))
    {
        number++;
    }
    else
    {
        if the number doesn't parse correctly, set it to zero
        number = 0;
    }
    number = (number > max) ? 1 : number;
    return string.Format("PC{0:D9}/POS", number);
}

//if the number doesn't parse correctly, set it to zero;