Benjamin as Jannet has mentioned, if you are using prestashop 1.7.6.7 you need to use PHP version not great than 7.2. Presta is going to support PHP 7.3 since 1.7.7 version and PHP 7.4 since 1.7.8 version. You can find more at this link https://devdocs.prestashop.com/1.7/basics/installation/system-requirements/#php-requirements
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Benkamin as Jannet has mentioned, if you are using prestashop 1.7.6.7 you need to use PHP version not great than 7.2. Presta is going to support PHP 7.3 since 1.7.7 version and PHP 7.4 since 1.7.8 version. You can find more at this link https://devdocs.prestashop.com/1.7/basics/installation/system-requirements/#php-requirements