qr-code-bundle | Bundle for generating QR codes in Symfony | QRCode Processing library

You should complain to the provider of that package for making a bad decision: the package enforces the installation of symfony/symfony (the full package of all Symfony components), while all current projects should depend on the individual components. That's what your project does: it installs each needed component, and not the whole framework.

And that's where stuff gets difficult for you: the full framework package symfony/symfony contains all components and lists them in the replaces section. Installing both the framework package and single components leads to problems and is not possible. But as that barcode package depends on the whole framework, and your own application depends on single components, the installation can not continue.

If you really want to use this package (there might be alternatives that structure their dependencies better), you can remove all Symfony components (those with the version constraint 4.3.*) from your composer.json and instead require symfony/symfony:4.3.*. Afterwards, that package should be installable