It's also possible to qualify the name in the ON CONFLICT clause, although I think you have to parenthesize it to do so:
... ON CONFLICT (("Lockers"."Uuid"))
This really needs to be confirmed and, if so, better documented on the INSERT page, since adding a schema to the index_column_name does not intuitively turn it into an index_expression which is what the syntax diagram says requires an extra pair of parentheses.
That of course should be adding a table qualifier to the column, not a schema qualifier. I get why INSERT doesn't really care, the source table is never in question in SQL-land, but as we see here there is a valid need for it in plpgsql.