Impossible to access an attribute ("seoCover") on a null variable.
{% if product.template == "shop-product" %}
{% set variation = shop.getLowcostProductVariation(product) %}
{% else %}
{% set variation = product %}
{% endif %}
{% set cover = variation.seoCover %}
{% set cover = cover.toFile() %}
{% set cover_url = cover.url() %}
{% if cover is not empty %}
{% set cover_url = cover.cropUrl(400, 400) %}
{% endif %}
<div class="relative">