bundler problem rails app isn't deploying to heroku

i tried deploying my rails app on heroku from github and also from my terminal but i got this error

-----> Ruby app detected
-----> Compiling Ruby/Rails
 !
 !     There was an error parsing your Gemfile, we cannot continue
 !     Warning: the running version of Bundler (1.15.2) is older than the version that created the lockfile (1.16.5). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
 !     The path `/tmp/spree_simple_sales` does not exist.
 !
 !     Push rejected, failed to compile Ruby app.
 !     Push failed

this is my gemfile.lock file

RUBY VERSION
   ruby 2.5.1p57

BUNDLED WITH
   1.16.5

i dong know why it is still giving me bundler problem, how can i fix it please.