The multi-part identifier “TextBox1.Text” could not be bound in C# ASP.NET?

I’m doing a practice project for training; my handler has specifically forbidden paramaterization and security-oriented coding for now, in the interest of getting the basics down. That being said, I’ve got a gridview on my homepage with a hyperlink field that takes the user to a page where they can edit the row data in textboxes. The row is displayed by the “ProductId” column, as it is autoincremented and unique. The values display perfectly, so I know my query string is fine, but when I attempt to update using the button event, I get an error message that says

Fill Missing Dates In a Date-Sequenced in SQL using Tally Table

I have a table in database with numbers of tenants, each tenant lists a record of their sales per date. There are instance where in a tenant has NO SALES in particular date/s, therefore the date with no sales has NO RECORD in the table breaking a proper date sequence. Please see the sample table for illustration below: