Installing Mollie payment plugin

Hi,

This indicates a fatal error.
The first thing ot do when you have a page where the processing stops in the middle like this or is completely white is to turn on the "error reporting" and "debug mode" settings of the Joomla configuration. That way, it will display an error message on that page which will indicate where the problem comes from.
Now, as the payment plugin is developed by a third party developer, I would recommend to directly send your request for help to them:
skyspider.com.au/#contact
That way, they will be able to provide a solution to you adequately.