Constraint error while creating a table: "Constraint specification not allowed here"

You syntax is malformed in several ways. Assuming the primary key in the funtom_employee table is called emp_id, you can either define the constraints in-line:

create table funtom_customer
(
  cust_id         number(3) primary key,
  cust_name       varchar2(50) not null,
  cust_contact    varchar2(50), 
  cust_acmanager  number(3) references funtom_employee (emp_id),
  cust_addr1      varchar2(50),
  cust_addr2      varchar2(50),
  cust_addr3      varchar2(50),
  cust_addrpc     varchar2(10)
);

... which will cause the system to generate the constraint names, or specify the, out-of-line as named constraints:

create table funtom_customer
(
  cust_id         number(3),
  cust_name       varchar2(50) not null,
  cust_contact    varchar2(50), 
  cust_acmanager  number(3),
  cust_addr1      varchar2(50),
  cust_addr2      varchar2(50),
  cust_addr3      varchar2(50),
  cust_addrpc     varchar2(10),
  constraint pk_funtom_customer primary key (cust_id),
  constraint fk_funtom_customer_acmanager foreign key (cust_acmanager)
    references funtom_employee (emp_id)
);

You can also create table and add the constraints afterwards, as @Thomas has shown.

You can name a not-null constraint but it's unusual; you can also have a named constraint that checks the column is not null, but then it wouldn't be shown as not nullable in the data dictionary (e.g. when you describe the table).

create table Funtom_customer
(
cust_ID       number(3) not null,
cust_Name     varchar2(50) not null,
cust_Contact      varchar2(50), 
cust_Acmanager    number(3)   ,
cust_Addr1    varchar2(50),
cust_Addr2    varchar2(50),
cust_Addr3    varchar2(50),
cust_Addrpc   varchar2(10)
)
;


alter table Funtom_customer add constraint pk_cust_ID  primary key (cust_ID);
alter table Funtom_customer add constraint fk_cust_Acmanager foreign key (cust_Acmanager) references funtom_employee(referenced_col);