I can see from your screenshot you are on the full Pause plan. On this plan, you don’t have access to your admin, so that’s the reason why you aren’t able to navigate about. If you are wanting to have access to your Shopify admin, then I recommend the Pause and Build Plan instead, which will allow you to login, work on your store, and continue to have customers visit your storefront. The differences between the two plans are detailed in this Help Doc.
That being said, the Pause plan allows you to pause your store for up to 3 months without any subscription fee charges, so I understand why you are concerned about an invoice for $50! How were you notified of the charges? Was it via an email? Or did you see the charges on your credit card statement?
Either way, I’m happy to look further into it for you. I’m sending you an email so that I can view the billing details of your store to locate that invoice and we can get some clarification of what the charges are for. Please keep an eye out for it, and reply at your earliest convenience so we can sort this out for you ASAP.