Re: UPDATE from CTE syntax error

https://www.postgresql.org/docs/10/static/sql-update.html
"column_name

    The name of a column in the table named by table_name. The column name can be qualified with a subfield name or array subscript, if needed. Do not include the table's name in the specification of a target column — for example, UPDATE table_name SET table_name.col = 1 is invalid.
"

So it should be:

SET letters = el.letters