Numbers: Syntax error

I apologise in advance, because I know this is a very basic question😊! I have an accounts spreadsheet for my new small business and have already successfully completed the calculations for March....but can't remember how I did it (and the formula shown doesn't really help). So, I have a column for 'Mileage' in which I want to put the mileage for that day, and have it multiplied by 0.45 pence per mile. I click on the cell, and try the following formula: =F3*0.45

but each time I get a Syntax error that reads: "This formula can’t reference its own cell, or depend on another formula that references this cell."

Is it because I haven't yet entered the data (daily mileage) that I want multiplied? I know it's something very simple, but can't for the life of me work it out!

Any help (in ABC/123 language please!) will be very gratefully received! TIA