Deploying a sinatra app produced on a mac to Heroku

Question

I have built a simple sinatra web app on my macbook. The deployment to heroku fails with the logs below. The problem is that I've already implemented their suggested fix (bundle lock --add-platform x86_64-linux) and I still get the same error message.

Any suggestions as to next steps would be very welcome. Thanks in advance.

-----> Building on the Heroku-20 stack
-----> Determining which buildpack to use for this app
-----> Ruby app detected
-----> Installing bundler 2.2.33
-----> Removing BUNDLED WITH version in the Gemfile.lock
-----> Compiling Ruby/Rack
-----> Using Ruby version: ruby-2.7.5
-----> Installing dependencies using bundler 2.2.33
       Running: BUNDLE_WITHOUT='development:test' BUNDLE_PATH=vendor/bundle BUNDLE_BIN=vendor/bundle/bin BUNDLE_DEPLOYMENT=1 bundle install -j4
       Your bundle only supports platforms ["arm64-darwin-20"] but your local platform
       is x86_64-linux. Add the current platform to the lockfile with `bundle lock
       --add-platform x86_64-linux` and try again.
       Bundler Output: Your bundle only supports platforms ["arm64-darwin-20"] but your local platform
       is x86_64-linux. Add the current platform to the lockfile with `bundle lock
       --add-platform x86_64-linux` and try again.
 !
 !     Failed to install gems via Bundler.
 !
 !     Push rejected, failed to compile Ruby app.
 !     Push failed

*Update

My local repo, with x86_64-linux in Gemfile.lock, is out of sync with the remote repo on github that I have been trying to deploy from. Other files are out of date too. Concerningly, I have both pushed and pulled recently, as well as deleted the remote repo, created a new one and pushed again. Git is telling me that both the local and the remote repos are up to date with each other, when they are obviously not.

I've tried pushing directly to heroku from the command line but hit the same platform error as above.