Search notes:

SP2-0552: Bind variable … not declared.

The error SP2-0552: Bind variable … not declared is thrown in SQL*Plus when

A bind variable is used in an SQL statement without having declared that bind variable, or
When declaring a bind variable without a datatype

:xyz is not defined:

SQL> select :xyz from dual;
SP2-0552: Bind variable "xyz" not declared.

When declaring a bind variable in SQL*Plus, the datatype must be specified, otherwise the error SP2-0552: Bind variable … not declared is thrown:

SQL> var xyz
SP2-0552: Bind variable "xyz" not declared.

Inspecting the variable

SQL> var xyz number

SQL> var xyz
variable   xyz
datatype   NUMBER

Assigning a value to the bind variable

SQL> exec :xyz ;= 42

Using the bind variable:

SQL> select :xyz from dual;