This looks like a theme problem (The Firm theme with Wizy Panel), not a WP problem.
If you have purchased that premium theme, you should have support for it from the vendor.
See if maybe there is a discussion forum on their website.
If they tell you it's not a theme problem (although it seems to be), make sure you have a back-up of the original theme (without any modifications you might have made) and see what could cause that error; you should also check with your hosting provider in case you haven't modified anything and this error suddenly appeared.
The WP Community cannot usually help you with premium themes.