I'm remove one tabulation:
extends layout

block body 

h1 Example page - Home: #{postData}
p Hello, World!
      - foreach(i, post; postData)
          | #{i}. #{post}

And now i get error:
index.dt(7): Error: undefined identifier i, did you mean template
to(T)?
index.dt(7): Error: undefined identifier post
/opt/vibe/source/vibe/http/server.d(634): Error: template
instance
vibe.templ.diet.parseDietFileCompat!("index.dt",HttpServerRequest,"req",string[string],"postData")
error instantiating
source/app.d(32): instantiated from here:
renderCompat!("index.dt",HttpServerRequest,"req",string[string],"postData")
source/app.d(32): Error: template instance
vibe.http.server.HttpServerResponse.renderCompat!("index.dt",HttpServerRequest,"req",string[string],"postData")
error instantiating
Failed: 'dmd' '-debug' '-g' '-w' '-property' '-g'
'-I/opt/vibe/source' '-Isource' '-Jviews' '-Isource' '-Jviews'
'-L-levent_pthreads' '-L-levent' '-L-lssl' '-L-lcrypto' '-v'
'-o-' 'source/app.d' '-Isource'