Creating a custom Percentage column in Direct Query mode

I don't know you are going to be able to do this with Direct Query. It has limitations, but if you simplify your DAX it might help.

Try the following measure. This will return the total percents correctly:

Percent of Account ID = 
VAR varCategory = SELECTEDVALUE('Sales'[Category])
VAR varAccountID = SELECTEDVALUE('Sales'[AccountID])
VAR varTotal = 
    CALCULATE(
        [Sales Amount],
        ALLEXCEPT('Sales', 'Sales'[AccountID])
    )
VAR varCategorySales =
    CALCULATE(
        [Sales Amount],
        'Sales'[AccountID] = varAccountID,
        'Sales'[Category] = varCategory
    )
VAR Result = DIVIDE(varCategorySales, varTotal, BLANK())
RETURN
    Result

Then for just the Pen percent, use this:

Pen Percent = 
CALCULATE(
    [Percent of Account ID],
    KEEPFILTERS( Sales[Category] = "Pen" )
)

I used a matrix, and I got rid of the totals in Power Query. Let the visual give you the totals.

Get the basics working first and see if the engine can generate the SQL code necessary for Direct Query. Then you can add IF statements or ISINSCOPE statements to get the visual like you want.

However, as a rule, I strongly advise against Direct Query. It has it uses, but import is where DAX works best, and if this were a multi-million record dataset, I don't care what DAX you write that gives you the results you want, it will be 3-10 times slower than an import model.