Not declared or inaccessible due to protection levels - VS 2017 errors

Hello

I am just wondering why, if I have these lines of code in my aspx file

<asp:label id="LblDate" runat="server" cssclass="dateStyle" text="Label">

<asp:panel id="pnlFormFields" runat="server">

<asp:panel id="pnlThankYouMessage" runat="server" cssclass="panel" visible="False">

why I would get the following red underlined errors (VS 2017) in my corresponding aspx.vb file:

pnlFormFields.Visible = False
pnlThankYouMessage.Visible = True 
LblDate.Text = ReturnDate()

Not declared or inaccessible due to protection levels.

What should I have, please?

Thanks!