If you want to insert Python values into a SQL database, just naming the Python variables in the SQL statement is not enough. The SQL database instead thinks you wanted to insert values taken from the table or another query instead.
Use SQL parameters instead, and pass in the actual values:

params = (userName, password, confirmPassword, firstName, lastName,
          companyName, email, phoneNumber, addressLine1, addressLine2, 
          addressLine3, zipCode, province, country, regDate)

c.execute("INSERT INTO People VALUES (NULL, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)", params)

The NULL value is for the p_ID primary key column; the alternative is to name all the columns you want to insert values for.
https://www.e-learn.cn/topic/482719