Can't add a foreign key constraint in a table on Postgresql
In the table centresdevote you have a row where the value of objectid is 10. As you are trying to create a FOREIGN KEY CONSTRAINT there is a check that there is a matching value in the column objectid in the table prefectures for all the rows in centresdevote. In prefectures there is no row with objectid = 10.

I suggest you read The PostgreSQL Documentation - 5.3.5. Foreign Keys and the W3Schools page about foreign keys. The whole point of the foreign keys is that they limit what you can insert into the field. From PostgreSQL Documentation:


  A foreign key constraint specifies that the values in a column (or a group of columns) must match the values appearing in some row of another table. We say this maintains the referential integrity between two related tables.


Adding foreign key is not mandatory and you can do every kind of query without it.