How do I fix these requirement conflicts?
drupal/webform 5.15.0 seems to be locked below the v5.15.0 and thus blocking the drupal\webform_node update.

check if you have an explicit version number in your composer.json that conflicts with the desired version
On several occasions, I unblocked such situation by removing the docroot/vendor directory followed by a composer install command (and removing the composer.lock file if a new conflict arised)

composer.json is a list of modules with version constraints that is user-defined.
composer.lock is the list of currently installed modules generated by composer install. If composer.lock is present, the list of resolved dependencies will be installed if you do composer install. This allow to define and deploy a set of resolved dependencies. composer update will resolve the dependencies again and update composer.lock (thus the update). From my experience, it could happen that you have unresolved conflict between explicit version definition in composer.json and the composer.lock. So you could get rid of it, and also the whole vendor directory, and do a composer install from scratch again. If the installation fails again, it must be from a human-generated composer.json conflict.