You need to make sure you use single quote for the Text
property eg Text='yourstuff'
.
Then use double quotes inside your bind statement. The code in the bind must be vanilla c#, if it won't compile in a .cs file it won't compile inline either and single quotes mean a char in c#, not a string.
This works:
<asp:TextBox ID="txtPassportNumber" runat="server"
Text='<%# string.Format("{0}{1}", "######", ((TSAPassenger) Container.DataItem).Passport.DocumentNumber.Text.Remove(0,6)) %>'
Enabled="<%# IsOutsideTenDayCutoff %>"></asp:TextBox>
Notice the single and double quotes. You should be able to copy and paste it as is.