Google Sheets conditional formatting based on cell above it

follow this steps

select cell b1

set conditional Format, say for Column B

apply to Range: B2:B (>it will later correct to B2:B999 or max rows)

Format Rules

Custom Formula: =not(isblank(b1)) >>fill RED

...done

if for some reason, like not selecting B1 at the beginning the formula (upon inspection) is different, pls correct to ...B1...

Try this custom formula: =not(isblank(offset(A1,-1,0)))

The only problem that I can see is that A1 is automatically highlighted, regardless of any value in any other cell