When you refer to whole datarange (i.e. all except header row) of named Table, you use Table name as reference. I.e. when you defined Table tData, then in formulas tData refers to data in this Table;
To refer to data in specific column in Table outside of this table, the reference will be like tData[ColumnHeader];
To refer to data in specific column in Table from same Table, the reference will be like [ColumnHeader];
To refer to cell in same row in same Table the reference will be like [@ColumnHeader].
(So @Selected refers to cell in column Selected positioned in same row as formula.)

There is more - try to google for "excel table formula syntax".

Btw, when you have formulas which use Table formula syntax, and you change Table name, or column header - this change is applied to all formulas using Table refernces automatically.

As about error you formula returns - post here the example workbook with faulty formula (NB! A small one!)! Otherwise it is difficult to help with this.

Edit: When using Table syntax, try to avoid spaces in column names. Otherwise you have to use much more squared brackets in formula, and formulas are more difficult to read. And with longer formulas, excel will force line breaks into formula field at top - so unless you make the formula field higher, you see only a part of formula at once.