It seems that GridDB´s REST API doesn't currently support a key-value pair to set a column´s nullity, such as "nullable": False. A workaround for that is to use the query editor of GridDB´s management website to create the table, using a regular CREATE TABLE SQL statement:

Create table Orders (
Order_ID INTEGER NOT NULL PRIMARY KEY,
Order_Date TIMESTAMP NOT NULL,
Client_ID INTEGER NOT NULL,
Order_NF STRING NOT NULL)