It looks like Drupal 10.1.0-rc1 updated the dependency version of 'guzzlehttp/promises' from 1.5 to 2.0.
Unfortunately, 'aws/aws-sdk-php' used by some modules is not compatible with 'guzzlehttp/promises’ 2.0, and this blocks the update to 10.1 stable (10.1-alpha does install).
The AWS SDK devs have acknowledged the issue, but it will not be addressed until at least August.
[~/www]# composer require drupal/core-recommended:10.1.0 drupal/core-composer-scaffold:10.1.0 drupal/core-project-message:10.1.0 --update-with-all-dependencies
./composer.json has been updated
Running composer update drupal/core-recommended drupal/core-composer-scaffold drupal/core-project-message --with-all-dependencies
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.