Update from Shopware 5.0.0 to 5.5.4 under PHP 7.1 possible?

Shopware 5.0.0 is not compatible with php7.1.

You need to make the update with php7.0 (e.g. in a local environment). Perhaps you can try the manual update package: https://de.shopware.com/download 
Shopware 5 - Update Guides - Updating Shopware