A formula refers back to its own cell, directly or indirectly.
Fix this with AIA circular reference warning appears when a formula depends on its own result — a loop Excel can't resolve, so it returns 0 or a warning.
Find the loop
Formulas → Error Checking → Circular References lists the offending cell.
Exclude the cell
Adjust the range so the formula no longer includes itself.
Rarely — only with iterative calculation enabled (File → Options → Formulas). Otherwise, fix the loop.