even though they said Tested with Laravel 5.5.* it in the readme,composer.json says it requires both
"laravel/framework": "5.4.*",
"illuminate/support": "5.4.*",
so if the package's composer.json says it should be Laravel 5.4.*. You are using laravel 5.5 so it's incompatible.
here is the rpsimao/invoicexpress-api composer.json file of tag 0.5.6. see Laravel version
{
    "name": "rpsimao/invoicexpress-api",
    "description": "Laravel Package to interact with InvoiceXpress API",
    "type": "library",
    "keywords": [
        "laravel",
        "rpsimao",
        "invoicexpress-api",
        "invoicexpress",
        "api"
    ],
    "homepage": "https://github.com/rpsimao/invoicexpress-api",
    "license": "MIT",
    "authors": [
        {
            "name": "Ricardo Simão",
            "email": "ricardo.simao@upgrade.pt"
        }
    ],
    "require": {
        "php": "^7.0",
        "guzzlehttp/guzzle": "^6.2.1",
        "guzzlehttp/psr7": "^1.4.1",
        "guzzlehttp/promises": "~1.0",
        "laravel/framework": "5.4.*",
        "illuminate/support": "5.4.*",
        "spatie/array-to-xml": "^2.5"
    },
    "autoload": {
        "psr-4": {
            "rpsimao\\InvoiceXpressAPI\\": "src/"
        },
        "files":[
            "src/Helpers/invoicexpress-api.php"
        ]
    },
    "require-dev": {
        "phpunit/phpunit": "^6.1",
        "orchestra/testbench": "^3.4"
    },
    "autoload-dev": {
        "psr-4": {
            "rpsimao\\InvoiceXpressAPI\\Test\\": "tests/"
        }
    },
    "extra": {
    "laravel": {
        "providers": [
            "rpsimao\\InvoiceXpressAPI\\InvoiceXpressAPIServiceProvider"
        ],
        "aliases": {
            "InvoiceXpressClients": "rpsimao\\InvoiceXpressAPI\\InvoiceXpressAPIFacade"
           }
        }
    }
}
AFTER EDIT
So, currently it's not possible to use with Laravel 5.5 as Laravel 5.5 uses PHP >=7.0.0 too, this package uses Laravel 5.4 which uses PHP >=5.6.4.
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