We're aware of an issue that occurs when trying to install extensions though the Web Wizard via the Magento Marketplace. This issue is not specific to WeltPixel modules, and seems to be a more general issue related to the Marketplace.
Here's an example of what you may see when trying to install extensions this way:
This is currently being looked into by the Magento team, however, in the meantime, there is a workaround that you can use. The modules can be installed directly via Composer using the CLI. Extensions can be installed via the composer require command. For example, if I were to install the Rich Snippets extension, I would issue the following commands:
composer require weltpixel/m2-weltpixel-google-cards
php bin/magento setup:upgrade
The exact name of the extension can be found in the composer.json file that you can download from your Magento Marketplace account.