Operator is not unique

If I do ...
select 1::NUMERIC + 1::BIGINT;

I get ...
[42725] ERROR: operator is not unique: numeric + bigint Hint: Could not
choose a best candidate operator. You might need to add explicit type casts.

This error means I have more than one way to calculate that formula ?
Did I create that operator erroneously ?
If so, how can I see what I did and how to fix it ?

I tried but didn´t find any answer.
select * from pg_operator
join LATERAL ( select typname::text from pg_type where pg_type.oid = oprleft
) l(oprleft_name) on TRUE
join LATERAL ( select typname::text from pg_type where pg_type.oid =
oprright ) r(oprright_name) on TRUE
join LATERAL ( select nspname from pg_namespace where pg_namespace.oid =
oprnamespace ) n(namespace) on TRUE

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