Error while installing the module

niharika.s → made their first commit to this issue’s fork.

thanks @niharika.s for creating the patch. I applied the patch but still getting the same error but this time in CommonMark.php see:

Fatal error: Declaration of Drupal\markdown\Plugin\Markdown\CommonMark\CommonMark::__sleep() must be compatible with Drupal\Core\Plugin\PluginBase::__sleep(): array in /code/web/modules/contrib/markdown/src/Plugin/Markdown/CommonMark/CommonMark.php on line 98

simarly, the code should be updated in PhpMarkdown.php. After doing these changes directly, i gets another error:
Symfony\Component\DependencyInjection\Exception\InvalidArgumentException: The service "markdown_plugin_manager" tagged "plugin_manager_cache_clear" must not be abstract. in Symfony\Component\DependencyInjection\ContainerBuilder->findTaggedServiceIds() (line 1345 of /code/vendor/symfony/dependency-injection/ContainerBuilder.php).

Check that the problem hasn't been addressed in the drupal-11 compatibility task https://www.drupal.org/project/markdown/issues/3438472 📌 Automated Drupal 11 compatibility fixes for markdown Active

I see that this issue is addressed in 📌 Automated Drupal 11 compatibility fixes for markdown Active , so closing this as a duplicate.

See this line from MR !37:
https://git.drupalcode.org/project/markdown/-/merge_requests/37/diffs?fi...