ASP.Net Error: textbox deos not exist

Hello,

Thank you for the answer of the title question as the above.  It's soved.  But I have the textbox (next to save button) issue:  Microsoft Visual Studio Professional 2017 editor does not know the text box object (after 20 minutes running the program.) I got error: textbox deos not exist (but it's still display in the form of web page). So, I keep doing delete the text box object then add it in the form again.  It works for 20 minutes running program then the error text box does not exist.... again.

Plaese help.  Thank you.