It looks like you need to close the quotes.

IF(
{Date début package} != ''", {Date début package},
IF(
{Date début expl 23} != ''", {Date début expl 23},

Hello and thank you for your feedback, this does not work I am attaching a view of the table Thank you and have a good day

https://airtable.com/appYGvpIAdS7m5Hab/shryMIhCEvJUzIFYG

What are you expecting to see as a result?

Good morning and thanks for your help.

Here is my expectation.

If cell "A" is not empty and cell "B" is empty then cell "C" takes the value of cell "A"
and if cell "A" is empty and cell "B" is not empty, then cell "C" takes the value of cell B.
If in cell "C" there is the value of cell "A" then you must add: "dateadd", "Number of days of Activity (from Planning 2024", 'days'

https://airtable.com/appYGvpIAdS7m5Hab/shr6gqkEP8FOIlHU2

while waiting for your back have a good day

As described:

If in cell "C" there is the value of cell "A" then you must add: "dateadd", "Number of days of Activity (from Planning 2024", 'days'

can never be true since cell "A" is a date and cell "C" is a number.  

Hello, thank you for your feedback

I got the solution, here is the formula:

IF(AND({A} != '', {B} = ''), DATEADD({A}, {ro}, 'days'), IF(AND({A} = '', {B}! = ''), {B}, IF(AND({A} != '', {B} != ''), {A}, BLANK())))
Good day

Great, good day to you as well.