Alright, it´s obvious that Trichocereus harrissima is not an accepted species. The name probably came from a seed or plant seller who sold this plant at some point in the past. I list it as a nomen nudum here, which means that it is a bad name that it is a “nude name” that lacks a formal description. This plant is actually Trichocereus purpureopilosus, which was described under that name and is extensively used in hybrid culture.
Trichocereus purpureopilosus is an amazing cactus with a beautiful appearance and flowers. The flowers are white in color and you can find out more about it here on the page: I wrote a whole lot more about this species on the page about it.
Synonyms: Trichocereus brevispinulosus, Trichocereus purpureopilosus.
I am currently not sure where the name Trichocereus Harrissima came from but I know for a fact that it originated after it was sold under that name. The name could be a reference to the genus Harissia, but it is very different to most plants from that genus.
These photos were provided by K. Trout / Troutsnotes.com.
Pics: Trout of Troutsnotes.com