Please test this in a copy of your workbook.
This should get you some way closer to what you want but we may need to find out more about what you want to happen when/if values in column K are changed or deleted.
At the moment, pretty much every time you change or delete a value (or multiple values) in column K the corresponding value(s) in column M will be deleted, but I'm not sure if that is what you want. Do some testing and come back with more details if modifications are required.
To implement ..
1. Right click the sheet name tab and choose "View Code".
2. Copy and Paste the code below into the main right hand pane that opens at step 1.
3. Close the Visual Basic window.
4. Make various changes in the sheet and observe results.
<font face=Courier New><br><SPAN style="color:#00007F">Private</SPAN> <SPAN style="color:#00007F">Sub</SPAN> Worksheet_Change(<SPAN style="color:#00007F">ByVal</SPAN> Target <SPAN style="color:#00007F">As</SPAN> Range)<br> <SPAN style="color:#00007F">Dim</SPAN> KChanged <SPAN style="color:#00007F">As</SPAN> Range, Cel <SPAN style="color:#00007F">As</SPAN> Range<br> <SPAN style="color:#00007F">Dim</SPAN> val<br> <br> <SPAN style="color:#00007F">Set</SPAN> KChanged = Intersect(Target, _<br> Columns("K"), _<br> Rows("2:" & Rows.Count))<br> <SPAN style="color:#00007F">If</SPAN> <SPAN style="color:#00007F">Not</SPAN> KChanged <SPAN style="color:#00007F">Is</SPAN> <SPAN style="color:#00007F">Nothing</SPAN> <SPAN style="color:#00007F">Then</SPAN><br> Application.EnableEvents = <SPAN style="color:#00007F">False</SPAN><br> <SPAN style="color:#00007F">For</SPAN> <SPAN style="color:#00007F">Each</SPAN> Cel <SPAN style="color:#00007F">In</SPAN> KChanged<br> val = Cel.Value<br> <SPAN style="color:#00007F">With</SPAN> Range("M" & Cel.Row)<br> <SPAN style="color:#00007F">If</SPAN> val = vbNullString <SPAN style="color:#00007F">Then</SPAN><br> .Clear<br> <SPAN style="color:#00007F">ElseIf</SPAN> IsNumeric(val) <SPAN style="color:#00007F">Then</SPAN><br> .Validation.Delete<br> .Value = "ok"<br> <SPAN style="color:#00007F">Else</SPAN><br> .Clear<br> <SPAN style="color:#00007F">With</SPAN> .Validation<br> .Delete<br> .Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, _<br> Operator:=xlBetween, Formula1:="=$Y$3:$Y$4"<br> .IgnoreBlank = <SPAN style="color:#00007F">True</SPAN><br> .InCellDropdown = <SPAN style="color:#00007F">True</SPAN><br> .InputTitle = ""<br> .ErrorTitle = ""<br> .InputMessage = ""<br> .ErrorMessage = ""<br> .ShowInput = <SPAN style="color:#00007F">True</SPAN><br> .ShowError = <SPAN style="color:#00007F">True</SPAN><br> <SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">With</SPAN><br> <SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN><br> <SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">With</SPAN><br> <SPAN style="color:#00007F">Next</SPAN> Cel<br> Application.EnableEvents = <SPAN style="color:#00007F">True</SPAN><br> <SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN><br><SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN><br></FONT>