When going to the checkout, this error occurs:
Fatal error: Class 'Detection\MobileDetect' not found in ... app/code/Netresearch/OPS/Helper/MobileDetect.php:45
The class cannot be found because the module was not installed properly.
Most likely, a drop-in installation was attempted (unpacking the module package files), but this doesn't work in Magento 2 without further ado.
Please follow the installation instructions for Composer from this article, section "Magento 2".