Why can't we use the word "sour" in this context?

[Samuel]

English Grammar Error, Intermediate level

ESL/EFL Test #7 "Her latest book", question 8

I like most coffees very much but I'm not too sure about this one. It seems much too sour to me.

(a) most
(b) about
(c) seems
(d) sour

English Grammar Error, Intermediate level

ESL/EFL Test #7 "Her latest book", answer 8

I like most coffees very much but I'm not too sure about this one. It seems much too bitter to me.

Correct entry: bitter
The error was: (d) sour

You have not found the error.
I like most coffees very much but I'm not too sure about this one. It tastes much too sour to me.
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why can't we use the word "sour" in this context?

Samuel

[Mister Micawber]

.
It is not an appropriate taste for coffee. Some sour things: cream, lemons and limes, old milk, grapes, dough, and sweet-and-sour sauce.

Sour: 'causing or characterized by the one of the four basic taste sensations that is produced chiefly by acids <sour pickles> -- compare BITTER, SALT, SWEET'.
.
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber