Welcome to the forum.
Some suggestions:
Use only letters and numbers (exception is the underscore) for object names.
Do not use spaces, punctuation or special characters in object names.
Do not begin an object name with a number.
Do not use Look up FIELDS, multi-Value fields or Calculated fields in tables.
You have fields "CIF/DNI/NIE". This is bad because of the special characters (the "/"). Maybe use "CIF_DNI_NIE"
You have a couple of fields with parenthesis in the field name - Also bad. (UnitWeight(Grams)) Maybe use "UnitWeightGrams" or "UnitWeight_Grams".
What is the field type for "CIF/DNI/NIE" in the "Customers" table? Text or Autonumber?
See
Microsoft Access Tables: Primary Key Tips and Techniques
Autonumbers--What they are NOT and What They Are