The RE [A-z] also includes the characters [, \, ], ^, _, and `. The RE [[:space:]] contains all whitespace characters; I'm guessing that you just want a space character instead. And, if you're...
Without modifying the shell variable:name="Matthew Lewis (writer)"
awk -v na="$name" 'BEGIN {gsub(/\(|\)/,"\\\&", na)} $0 ~ na' file.txt
Matthew Lewis (writer)