'The Temple' by HP Lovecraft was written in 1920 but the U29 U-boat wasn't built until 1936, can anyone explain?

Which U29 are you referencing?

https://en.wikipedia.org/wiki/SM_U-29_(Germany) - Launched 1913

edit - plot summary

The story is narrated as a "found manuscript" penned by Karl Heinrich, Graf von Altberg-Ehrenstein, a lieutenant-commander in the Imperial German Navy during the days of World War I. Altberg begins by declaring that he has decided to document the events leading up to his untimely end in order to "set certain facts" before the public, aware that he will not survive to do so himself.

I would presume, given the plot is during WW1, the book is referring to U-29 I have linked, as that was a WW1 submarine

Lovecraft was so racist he could see into the future

https://en.wikipedia.org/wiki/SM_U-29_(Germany)

vs

https://en.wikipedia.org/wiki/German_submarine_U-29_(1936)

(I haven’t read the story nor do I know anything about submarines so I could be wrong, I just did some googling)