SQL> connect u1@pdb1sol122
Enter password: **
Connected.
SQL> create sequence test_seq start with 100 increment by 10;

Sequence created.

SQL> create table test_tbl
  2  (
  3  col1 number default test_seq.nextval,
  4  col2 varchar2(30)
  5  );

Table created.

SQL> 

SQL> connect scott@pdb1sol122
Enter password: *****
Connected.
SQL> create table test_tbl
  2  (
  3  col1 number default u1.test_seq.nextval,
  4  col2 varchar2(30)
  5  );

Table created.

SQL> 

grant select on u1.test_seq to scott;

SQL> connect scott@pdb1sol122
Enter password: *****
Connected.
SQL> create sequence test_seq start with 100 increment by 10;

Sequence created.

SQL> alter table test_tbl
  2    modify col1 default test_seq.nextval
  3  /

Table altered.

SQL>