/usr/local/lib/ruby/gems/2.4.0/gems/pdfkit-0.4.6/bin/pdfkit:29:in `download_wkhtmltopdf': File not found.. (RuntimeError)
from /usr/local/lib/ruby/gems/2.4.0/gems/pdfkit-0.4.6/bin/pdfkit:63:in `block (2 levels) in <top (required)>'
from /usr/local/lib/ruby/2.4.0/optparse.rb:1552:in `block in parse_in_order'
from /usr/local/lib/ruby/2.4.0/optparse.rb:1538:in `catch'
from /usr/local/lib/ruby/2.4.0/optparse.rb:1538:in `parse_in_order'
from /usr/local/lib/ruby/2.4.0/optparse.rb:1532:in `order!'
from /usr/local/lib/ruby/2.4.0/optparse.rb:1626:in `permute!'
from /usr/local/lib/ruby/2.4.0/optparse.rb:1648:in `parse!'
from /usr/local/lib/ruby/gems/2.4.0/gems/pdfkit-0.4.6/bin/pdfkit:73:in `<top (required)>'
from /usr/local/bin/pdfkit:23:in `load'
from /usr/local/bin/pdfkit:23:in `<main>'
when I ran "pdfkit --install-wkhtmltopdf":
how can I fix it?