An Amazon delivery guy rang me and told that he was waiting for me at my place and I replied by saying that "I won't be able to attend you before 12 pm" but I am a bit confused about whether I was correct in using "attend" .. I think it should be "attend to" i.e. "I won't be able to attend to you before 12 pm
According to the Lexico online dictionary "attend" can be used with a noun or pronoun as a direct object. Their definition and example are:
Escort and wait on (a member of royalty or other important person)
‘Her Royal Highness was attended by Mrs Jane Stevens’
This use of "attend" is quite rare and very specific in its meaning, normally "attend" without "to" is used with events or institutions as in "I attended the conference on Monday" or "My son has got over his cold, he was well enough to attend school today."
When used in the sense of "deal with" or "work on" "attend" is accompanied by "to", examples from Lexico being:
he muttered that he had business to attend to
and with slightly different meanings
the severely wounded had two medics to attend to their wounds
Alice hadn't attended to a word of his sermon
The problem with your use of it as described is that you were conferring VIP status on the Amazon delivery driver, which was probably not what you intended.
You could have said "I won't be able to attend to you until 12:00" but that can have the opposite effect of treating the driver as an object or a task which can sound insulting. I would probably have said "I'm afraid I can't get there until 12:00 at the earliest.