I got the same error today and I solved it with a little editing. Just do this:
Navigate to
vendor/symfony/translation-contracts/LocaleAwareInteface.php
and delete string keyword from the setLocale() function.
If you get this error after this:
Declaration of Symfony\Component\Translation\Translator::trans($id, array $parameters = Array, $domain = NULL, $locale = NULL) must be compatible with Symfony\Contracts\Translation\TranslatorInterface::trans(string $id, array $parameters = Array, ?string $domain = NULL, ?string $locale = NULL)
Just navigate to:
vendor/symfony/translation-contracts/TranslatorInteface.php
and delete all the string keywords from the trans() function
--EDIT--
If you don't want to edit vendor files, you can use this:
Open your composer.json file, and add this in "require"
"symfony/translation": "4.3.8",
Then open your terminal and type this:
composer update
config/app.php?