BUG #18222: Unexpected Error--Cannot delete from scalar

The following bug has been logged on the website:

Bug reference:      18222
Logged by:          Jinsheng Ba
Email address:      bajinsheng@u.nus.edu
PostgreSQL version: 16.1
Operating system:   Ubuntu
Description:

SELECT ('1'-to_hex(2))::BOOLEAN; -- ERROR:  cannot delete from scalar

This error looks strange to me as I do not understand what this error
means.

If I execute the following query, there is no error instead:
SELECT '1'-2; -- {-2}