It's a memory error.  Start by unplugging the system, disconnecting the power cable and holding the power button for 30 sec.  Remove and reinstall the memory module(s).  Does that solve the problem?

@ejn63​ how do I get inside the pc though

Depends on the case type  -- documentation here

https://www.dell.com/support/home/en-us/product-support/product/optiplex-780/docs