access to function - ORA-00904: invalid identifier - oracle

Question

I am new to oracle, please assist.
I have written a function similar to here in schemaB, with minor alterations.
When I run the query from above function, in schemaA, it runs fine and returns the sequence.
select schemaB.sequence_name_seq.nextVal from dual;
Running the function from schemaB ofcourse return the sequence as expected.
However, when I try to access the same function (containing the above query) from schemaA, it gives me an error:
"ORA-00904: invalid identifier"
I have granted EXECUTE privileges to userA of schemaA (confirmed from 'DBA_TAB_PRIVS' table).
Function:
create or replace Function nextSeq
(
tableName in VARCHAR2
)return NUMBER as
nextNum Number;
begin
EXECUTE IMMEDIATE 'select '||tableName||'_SEQ.nextval from dual' into nextNum;
return nextNum;
END nextSeq;
To Call:
select nextSeq('SCHEMAB.TABLENAME') from dual;

Answer 1

Here's how I'd do that: connected as SCOTT, I'm creating a sequence and a function; then I'll grant EXECUTE on the function to user MIKE:
SQL> show user
USER is "SCOTT"
SQL> create sequence dept_seq;
Sequence created.
SQL> create or replace function nextseq (tablename in varchar2)
2 return number
3 as
4 nextnum number;
5 begin
6 execute immediate 'select ' || tablename||'_seq.nextval from dual' into nextnum;
7 return nextnum;
8 end;
9 /
Function created.
SQL> select nextseq('dept') from dual;
NEXTSEQ('DEPT')
---------------
1
SQL> grant execute on nextseq to mike;
Grant succeeded.
SQL>
Connect as MIKE and use SCOTT's function:
SQL> connect mike/lion#xe
Connected.
SQL> show user
USER is "MIKE"
SQL> select scott.nextseq('dept') from dual;
SCOTT.NEXTSEQ('DEPT')
---------------------
2
SQL>
As you can see it works. When compared to your code, the difference is here:
You: select nextSeq('SCHEMAB.TABLENAME') from dual;
Me : select scott.nextseq('dept') from dual;
You shouldn't precede table name with the owner, but the function name.