Active File Path from text string

?

'=&'''N:\INVOICES\Store A\[12-30-19.xlsm]Cost'!$L$109 isn't a valid formula.

If you mean you have formulas returning text which look like external references, then unless the workbooks to which those external references pointed were all open in the same Excel session, the only way to turn those formulas into actual external references would be to copy those cells, paste-special as values then replace = with = in those cells (yes, replace = with itself, which has the effect of reentering the text constants as formulas). There's no other simpler way to do this, though if you need this often, you may want to create a macro in your PERSONAL.XLSB for this.

Sub textref2extref()
  Dim a As Range

  If Not TypeOf Selection Is Range Then Exit Sub

  With Application
    .ScreenUpdating = False
    .EnableEvents = False
  End With

  For Each a In Selection.Areas
    a.Value = a.Value
    a.Replace what:="=", replacement:="=", LookAt:=xlPart
  Next a

  With Application
    .ScreenUpdating = True
    .EnableEvents = True
  End With

End Sub