database design and links

Good day.
I'm trying to write my first program.
I have tried to create a database to use in our company for staff loans … I am always failing. The program says that my relationships are not conflicting and then I miss some parts to write everything in the next attempt and each attempt is another problem.

Can someone simply tell me all the basic table I need and what row goes in each and the links to the other table?
I made the table like this:
Customer (the rows are name, surname, customer, invoice)
Invoice (date, time, loan ID, invoice no)
Loan (loan ID, open amount, loan amount, total)
SEMrush

That's the way I'm trying, but now I'm not successful for 5 months. So I really tried all the ways I could think, except the right one.

Thank you for helping and enjoying your day.