Help installing Upfront Builder

Hello, I am new to WPMU DEV and have installed a few of your plugins on my main website (everythingtocelebrate.com)

I have now added a client’s website (eat-me-ltd.com) and the idea is to install upfront builder and design a website using this plugin.

However I cannot install Upfront or a theme or any other plugin for that matter.

I get a red bar across top of screen that says ‘Server error status: Internal Server Error [500]’ this seems to be the case for any plugin I try install.

Where am I going wrong, please help?

Best regards

Andrew