A circular reference occurs when a formula has a cell that points to itself. In the example below C3 has a formula =C3 so pointing to itself. To find a circular reference Use [Formulas] > Error Checking > Circular References > then click on the cell(s) that is showing.
Sometimes when you copy a sheet to another workbook, the old sheet will contain links and when you open you workbook you get.
So the challenge can be to find the link. Fortunately the solution is relatively simple. The offending cell will usually refer to another excel workbook
e.g. ='F:\_Data\2020Data\[2020_Invoice_v2.xlsx]Notes'!B37
so you can use the find command to search for ".xl"
Highlight the whole worksheet (click top left at intersection of rows & columns), the [Ctl]+[F] (find) then enter *.xl* in the "Find What" box and click [Find All]. All the offending links will be highlighted.