Excel formulae working in one excel sheet but not another one in exactly the same layout

Question

I was struggling to get excel to return a formula so I asked a colleague but took out all real data as HR-related. I'm trying to work out years and months for the length of service. So I sent it to colleague and then tried one final time to remove spaces at the start of the formula and it suddenly returned an answer.

Hurrah I hear you say ... so I copied it across to the real spreadsheet and but sadly nothing comes up except this error message

Error in Value

A value used in the formula is of the wrong data type.

Please help me, spent hours 😞

The cell I am trying to see answer is set to general, I seen someone say try Ctrl, shift and enter to paste but nothing pastes when I do this

This the formula

=DATEDIF(L4,N1,"y")&" years, "&DATEDIF(L4,N1,"ym")&" months"

L4 is the cell with star dates and N1 is =TODAY()

Thanks in advance for any help