How do i change the field of primary key in a record ?

If your record was created from a database table, you need to update the primary key in the database table. After that, perform a remapping of the record source in Appian. Once the remapping is complete, the changes will be reflected in the record type

This is the error iam getting when iam trying to give the primary key as a new field.

Hi skzahed_09 
If you have created a new field JOB_POSTING_ID after there are some entries inserted in the table, then while creating the field keep the default value as NULL(If not null then 0's will be stored to all the fields and above error will be shown) and apply the auto increment (Click on check box).

I tried your way but still i am getting the same error. 

Did you add JOB_POSTING_ID as a new field or changing the old field as a primary key? I can see that the field is not auto increment 
If it is then in Extra column we will get it as AUOT_INCEMENT
Like this.
So make it auto increment then your error will be resolved.

In the first i made the table and gave a primary key as company_name and then i had to add a new field as job posting id and give that field as the primary key for which i dropped the company name from primary key and till that i had no issues and when i was trying to make the job posting field as a primary key it showed error. I kept it null and also none and tried but it gave the same error. And coming to the auto-increment part i have no option where i can give that field an auto-increment. I would be pleased if i can get any kind of assistance in that area. 

In the first i made the table and gave a primary key as company_name and then i had to add a new field as job posting id and give that field as the primary key for which i dropped the company name from primary key and till that i had no issues and when i was trying to make the job posting field as a primary key it showed error. I kept it null and also none and tried but it gave the same error. And coming to the auto-increment part i have no option where i can give that field an auto-increment. I would be pleased if i can get any kind of assistance in that area. 

Click on change option from structure and click on check box under A.I (9th column before comments).

I completely deleted the field and added and new field and tried to give it A.I and set it as primary but this error was shown.

And i cant find the Auto-increment option while i click the change option, its available only when iam creating or adding a new field.