SUMIFS() is one way, but there's too much missing data to explain what's wrong here.
For a start, we can't see the column headings, so we have no idea which columns are C, F, and G.
Secondly, your formula references a 'Transactions' table, but we can't see that table. The table you show seems to have a title of 'April'. Maybe that's the problem...? you're referring to a completely different table?
Your second image also seems to show a table called 'April' but your description says it should be 'Monzo Summary.
Then, your formula references rows 3 through 77, but you're only showing half a dozen rows, so we can't see what other values are in the column that could be affecting the outcome.
FWIW, we can't tell which cell that formula relates to (no cell in the screenshot is highlighted).
IF we assume that the table reference is correct, let's look at SUMIF()
It takes three parameters, a range to search, a condition, and a range to sum for cells that match the condition.
I'll assume that we're looking at two different tables here, called 'Transaction' and 'Summary'.
The formula in Summary to count all 'Mortgage' paid-out values would be:
=SUMIF(Transactiion::D,"=Mortgage",Transaction::F)
which assumes that column Transaction::D relates to the 'Subcategory' column, and column Transaction::F is the Paid Out.
Ultimately, you might just save yourself a whole bunch of time and create a pivot table. They are designed to summarize data like this, in just a few clicks.
Select any cell in the Transaction table and choose the Organize -> Create Pivot Table... menu
Then, in the sidebar, tell the pivot table that you want the Subcategories in the Row, and the Paid-Out cells in the values (drag the field labels from the Fields section to the relevant Pivot section:
and you should get something like this:
