Fatal error when adjustment percent is NULL

Why do you have a NULL tax rate in the first place?

Did you define a custom tax type plugin? cause the LocalTaxTypeBase has this code:

        $order_item->addAdjustment(new Adjustment([
          'type' => 'tax',
          'label' => $zone->getDisplayLabel(),
          'amount' => $tax_amount,
          'percentage' => $percentage->getNumber(),
          'source_id' => $this->parentEntity->id() . '|' . $zone->getId() . '|' . $rate->getId(),
          'included' => $this->isDisplayInclusive(),
        ]));

Are you using the Avatax module by any chance? We should fix this, I realized we're not setting the "percentage" when adding the adjustment:

      $item->addAdjustment(new Adjustment([
        'type' => 'tax',
        'label' => $adjustments[$item->uuid()]['label'],
        'amount' => new Price((string) $adjustments[$item->uuid()]['amount'], $currency_code),
        'source_id' => $this->pluginId . '|' . $this->entityId,
      ]));

Shouldn't we try calculating the percentage? Klarna doesn't complain about a "0" tax rate? Is the tax amount 0 as well?

See https://www.drupal.org/project/commerce_avatax/issues/3138492#comment-13....

I'm pretty sure Klarna will crash if we pass a 0 tax_rate (in case a tax amount is positive), so I believe the proper fix is to make sure the tax adjustment has a percentage set on the tax adjustment, or the percentage should be calculated, so not committing this.

I've just stepped-into step-debugging mode to learn more about this issue and I believe this is related to how Adjustments are handled in general, meaning it may actually be an issue in commerce core.

from Drupal\commerce_order\Adjustment.php:

    if (!empty($definition['percentage'])) {
      if (is_float($definition['percentage'])) {
        throw new \InvalidArgumentException(sprintf('The provided percentage "%s" must be a string, not a float.', $definition['percentage']));
      }
      if (!is_numeric($definition['percentage'])) {
        throw new \InvalidArgumentException(sprintf('The provided percentage "%s" is not a numeric value.', $definition['percentage']));
      }
    }
...
    $this->percentage = !empty($definition['percentage']) ? $definition['percentage'] : NULL;

So if the tax rate resolves to "0" (which is valid tax rate in some tax zones) the code above will result in Adjustment::percentage member property set to NULL, causing the fatal error in question. However, IMHO this is to be solved in Commerce Core rather than here.

Created #3187199: Adjustment percentage member property cannot be set to 0