Neither of the two formulas work

You have to remember that select statements always return an array. So for the first one you need to do a few things:

let xDoc := first(select 'DOCUMENTOS');
let xFld1 := xDoc.'Avance Plan';
let xFld2 := xDoc.'Avance Real';
let i := (create AVANCES_PROY);
i.('Avance Plan' := xFld1);
i.('Avance Real' := xFld2)

You first need to put line 1 into a variable.

Then you need to tell Ninox that you want either the first or last record selected. You current statement has you selecting the entire Documentos table so you will need to be a bit more specific. Or just use the first or last array command.

Then in lines 2 and 3 you can use the new variable from line 1 to tell Ninox where to get the fields 'Avance Plan' and 'Avance Real'.

I would recommend using this way as you only do 1 select statement. The 2nd method does two so it is not efficient.

To do a series of dates you need the for loop command. Which means you need an array of the dates you want to create records for. One way is to create the array in the formula. I've done it another by creating a table that stores the dates you need, I called it dates. You had a table of dates in a previous version. I find the table method easier to manage as I can easily see the dates.

Then you just modify the button to include the for loop command and the bit about changing the DataDate in the FlecaCorte table so it can show up properly in Documentos. And it all works.

Give it a try and let me know if that is what you needed.