How to output an Entries Field value within a Matrix Field block
You get this error because {{ block.slideLink }} returns an array of EntryModels (even if you set your Entries field up to only allow 1 entry). So you need to add the first() method to get the first (and only) EntryModel that you are interested in:

{% set entry = block.slideLink.first() %}


Access the url property to return the URL of that entry:

{% if entry %}
    {{ entry.url }}
{% endif %}