Is it permissible to insert a null value into a column that is a primary key of a table?

Inserting a NULL value into a column that is a primary key is not allowed.

A primary key must uniquely identify each record and cannot contain empty or undefined values. Therefore, in most DBMSs, attempting to insert NULL into a primary key column will result in an error.

If you need to allow the absence of a value, you should use other mechanisms, such as unique keys that permit NULLs, or separate columns that can accept NULL.