Facing an error : table or view does not exist

Question

I am using insert statement and trying to insert data into the database table. I am using stored procedures.
But I am getting this error while doing so.

Message: ORA-00942: table or view does not exist ORA-06512

I checked if the tables/stored procedures are present or not and everything is in place. Also there is no typo in table names or in sp. If I run the part of SP from query editor it works fine but when I execute the entire SP it throws an error.

I tried the steps provided by Stephen but since I have logged in with the same user/owner when I run Grant command it gives me an error saying 'Cannot Grant/revoke on own'.
One more addition to this. I have a stored procedure SP1 in which I am using a select statement as

Select a from table_name where condition;

When I execute this seperately, it returns me some results. But when I execute sp it gives an error at the same line where it is written.

Can anyone help me out to resolve this issue. I am using SQL +.
Thanks in advance Vijay