These are the ten reasons why a catholic person can ask for the annulment of the marriage, I am not good at translating, but I will try.
https://www.abc.es/sociedad/20140220/abci-nulidad-matrimonial-liguria-201402191716.html
King Juan Carlos cannot request the annulment of the marriage.
1. Impediment of impotence in both men and women (canon 1084).
2. Impediment of link (canon 1085). If you were married previously and that bond has not been broken
3. Inhibition of consanguinity (canon 1091). Between two brothers (of the same father and the same mother), a marriage cannot be contracted because there is an impediment of natural law. In the case of first cousins, the bishop may dispense this impediment. .
4. Vice of consent due to lack of use of reason (canon 1095, 1º).
5. Vice of consent due to serious lack of internal discretion (canon 1095, 2º). It can occur in the case of a wedding couple who, after two years of relations, realizes that she is pregnant. When their parents hear the news, they pressure young people to marry even though they are not determined. If during the process it is possible to prove that one or both have not been internally free to express consent, that marriage can be declared void.
6. Nullity due to inability to assume the essential obligations of marriage due to causes of a psychic nature (canon 1095, 3º).
7. Error about the person (canon 1097). It occurred in the Middle Ages when the kings agreed to a marriage and the future spouses saw each other for the first time on the wedding day. There they realized that the person who had introduced himself was not the future husband or wife. Today it is difficult to produce.
8. Fraud to obtain consent (canon 1098). Being sterile is not a cause of marriage annulment if the other party is aware of this circumstance. On the other hand, if the person knows that she or he is sterile and hides it because if not the marriage is not celebrated, then it is a cause of nullity.
9. Simulation of marriage by exclusion of one of its essential properties --unity or fidelity, indissolubility and openness to life-- (canon 1101). This is the case of a person who simulates consent at the time of marriage. In her inner forum she is excluding some of the characteristics of marriage such as fidelity, indissolubility or procreation. For example, you are getting married but not with the intention of having children and your spouse does not know it. The problem in these cases is the evidence, but usually there are witnesses who have heard one of the parties express their true intentions.
10. Defect of form (canon 1108).This is when, for example, they were married before a priest who was not titled to celebrate the marriage..