Fatal error: Declaration of Drupal\views_data_export_phpspreadsheet\Encoder\XlsEncoder::encode(): string must be compatible with Symfony\Component\Serializer\Encoder\EncoderInterface::encode()

we don't support Drupal 9 anymore. it will create phpstan error in drupal 11 +

lazzyvn → closed merge request !6

Currently version 2.0.5 supports D9 per the module's homepage and the bug is valid.
You could probably create a new release that fixes this specific error and then another one that removes D9 support.

But of course that's up to you, as a maintainer.

you need only update your vendor phpoffice/phpspreadsheet to version 3

I have it updated. It's in 3.8.0

maybe your php version is 8.3+ it will check strict type of variable. I run drupal 9 with php8.1 it works fine

My complete list

PHP 8.1.31
Dcore 9.5.11
views_data_export 1.5.0
views_data_export_phpspreadsheet 2.0.5
symfony/serializer v4.4.47 (part of it is \Symfony\Component\Serializer\Encoder\EncoderInterface)
phpoffice/phpspreadsheet 3.8.0

Reverting to v1.x, especially to 1.6.0 you released today, the error goes away, since the signature has now mixed $format instead of string $format in v2, where string $format gets us this error.
So, it's still OK for v1 but if anyone in D9 upgrades to v2(2.0.5), they will probably get that error