You appear to have an extra quote in your query. The following should work for you:
vSQl := 'select toValueText(a.code, a.descr) from
(select currency_code code, des1 descr '||'from sy_curr_code ) a ';
Note the quote before your closing parenthesis has been removed.
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solved Receive an unexpected symbol error in my PLSQL query [closed]