Do I get it right that the third LOAD statement tries to load from two tables at the same time? a LOAD statement has only one source.
Also, it seems that the third LOAD has no source specification at all... You should at least specify a FROM..., RESIDENT... INLINE, AUTOGENERATE or FROM FIELD clause. Or add another LOAD/SELECT to create a PRECEDING LOAD.
NOM_PRENOM_Equal will not load as that table doesn't have any source (FROM, Resident...). If you use resident it will load from one previously loaded table so you'll need to use a join or applymap.
As you're trying to load records where Nom and Prénom are equals you can try:
Mapping LOAD Nom & '#' & Prénom, ID FROM...; //Dou table
[Nom] as [Nom dou],
Prénom as [Prénom dou]
Where not IsNull([ID dou];
ApplyMap('MAP_DOU', [Nom] & '#' & Prénom, Null()) as [ID dou]
FROM ... // Ref table
Not tested, hope this helps.