Solved - Error with base theme

There's more info about this issue here for bootstrap5:
https://www.drupal.org/project/bootstrap5/issues/3337690 💬 Upgrade 2.x to 3.x Fixed

In practice you can get things working by running updates from the user interface as drush updb doesn't work.
If you aren't logged in then you will need to set update_free_access in settings.php.
(You don't have to make stable9 visible).

Other help on that thread says that it works using drush 11 but not drush 10.

Thank you chriscant!

Just for the heck of it I did the update and ran drush updb.
It croaked of course.

public_html$ composer require 'drupal/bootstrap4:^3.0'
./composer.json has been updated
Running composer update drupal/bootstrap4
Loading composer repositories with package information
Info from https://repo.packagist.org: #StandWithUkraine
Updating dependencies
Lock file operations: 0 installs, 1 update, 0 removals
  - Upgrading drupal/bootstrap4 (2.1.16 => 3.0.1)
Writing lock file
Installing dependencies from lock file (including require-dev)
Package operations: 0 installs, 1 update, 0 removals
  - Downloading drupal/bootstrap4 (3.0.1)
  - Upgrading drupal/bootstrap4 (2.1.16 => 3.0.1): Extracting archive
Package doctrine/reflection is abandoned, you should avoid using it. Use roave/better-reflection instead.
Package symfony/debug is abandoned, you should avoid using it. Use symfony/error-handler instead.
Package webmozart/path-util is abandoned, you should avoid using it. Use symfony/filesystem instead.
Generating autoload files
Hardening vendor directory with .htaccess and web.config files.
52 packages you are using are looking for funding.
Use the `composer fund` command to find out more!
Cleaning installed packages.
No security vulnerability advisories found
public_html$ drush updb
 ------------ ------------------------- ------------- -------------------------
  Module       Update ID                 Type          Description
 ------------ ------------------------- ------------- -------------------------
  bootstrap4   install_stable9           post-update   Enable new theme:
                                                       stable9.
  bootstrap4   uninstall_stable_classy   post-update   Uninstall old themes if
                                                       present: stable and
                                                       classy.
 ------------ ------------------------- ------------- -------------------------


 Do you wish to run the specified pending updates? (yes/no) [yes]:
 > yes

>  [warning] Post update function bootstrap4_post_update_install_stable9 not found in file bootstrap4.post_update.php
>  [error]  Update failed: bootstrap4_post_update_install_stable9
 [error]  Update aborted by: bootstrap4_post_update_install_stable9
 [error]  Finished performing updates.

Then I hit https://www.mysite.com/update.php and it updated properly without errors.
Thanks again.

Thanks everyone!

Automatically closed - issue fixed for 2 weeks with no activity.

Followup: Note, upgrading to Drush v11+ fixes the problem with the post_update hook, since that is a feature introduced for themes in drush 11. This should be added as a minimum requirement in the theme's composer.json file.

Can the documentation for migrating from 2.x to 3.x be updated? This line doesn't seem to work for anyone:

Run updates via drupal interface if using drush 10 or less.

I feel like the following would be more clear.

Upgrade to branch 3.0.x

If using Drush 11 (recommended)

• Run drush updb
• If errors already appeared, use either drush theme:uninstall claro stable and/or drush theme:install stable9 depending of what php error you'll get.

If using Drush 10

• Unhide stable9 by commenting out the "hidden: true" line in web/core/themes/stable9/stable9.info.yml
• Run database updates via the Drupal interface to complete migration.

Thanks for sending though changes @davedg629. 🧁
I updated theme page.

No problem. We should add one thing for the Drush 10 instructions. You need to actually install the stable9 theme after unhiding it. Below are the updated instructions that include this step.

Upgrade to branch 3.0.x

If using Drush 11 (recommended)

• Run drush updb
• If errors already appeared, use either drush theme:uninstall claro stable and/or drush theme:install stable9 depending of what php error you'll get.

If using Drush 10

• Unhide stable9 by commenting out the "hidden: true" line in web/core/themes/stable9/stable9.info.yml
• Go to Appearance and install the Stable9 theme.
• Run database updates via the Drupal interface to complete migration.

Thanks @dadderley #3
"Then I hit https://www.mysite.com/update.php and it updated properly without errors." is ok for me.

Merci :-)