Power query append?

Hi guys
I'm not sure the way to take for this problem that i have. I hope you can help me.
I have a table with all sales, as well as a table of all purchases.
I want to check every item, sub item, family and sub family by cost center of every product and, of course, do some calculated fields with those fields.
I have the table of sales, on a table and added to data model as well as the table purchases. I also have the table for products (unique records).
I would like to have those first 2 tables in the same, so i could relate with product table later. But when i try to append them it gives me an error on more than 700 records and i can't see none.
The fields are all the same in both tables.
What am i doing wrong? Do you suggest me to do it in another way?
Thank you so much for your replies.
Best regards
Pedro

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