The AutoComplete feature of Excel is pretty handy. When you are entering information into a cell, it automatically provides you with a list of the previous entries in the column that match what you’ve typed. Thus, if you type the letter T, then it lists all those entries starting with T. When you type the second letter, R, then it reduces the list to all those entries starting with TR.
There is a limit to AutoComplete, however: It will only search for matches in the column until it hits a blank cell. For instance, if you have values in the cells in A3:A17 and in A19:A26 (cell A18 is blank), then when you start to enter information in cell A27, only the entries in the range A19:A26 are used to display the AutoComplete list.
If you want to have Excel use everything in the full range (A3:A26) as fodder for the AutoComplete list, then there is no way around it-you will need to enter something in the blank cell (A18). A good choice is, perhaps, a single character, like a minus sign, an x, or a z. Select A18, type your character, and then press Enter. The cell now contains something, so AutoComplete will reference the entire range (A3:A26) when entering information into A27.