TRUE and FALSE don't work within SUM()

I've had success with COUNTIFS over a range where he condition is TRUE

=COUNTIF(D2:D51,TRUE)

You can try prefixing the range with -- and entering as an array. The -- will convert the booleans into their integer equivalents:

=SUM(--(A1:A4))

Per the documentation on the SUM function:

If an argument is an array or reference, only numbers in that array or reference are counted. Empty cells, logical values, or text in the array or reference are ignored.

If you want to use sum on the true/false field you could multiple the true/false condition by 1 so that it evaluates to either 0 or 1. Then your sum function would work on the column.