swimming training creation

Hello, when I want to create a 50m swimming pool training, I cannot choose a length of 25m (must be greater than or equal to 50m). So if I want to do a 50m split into 25 + 25 with a difference swim I can't?

If you want to make 25m lenghts (even if it is 50m pool), just select the 25m pool.

Something else in movement selection, is there the back crawl but not the crawl itself?

It is called Backstroke ("Dos crawlé" en français). It is there as an option, but I do not understand why you are asking whether it is there, when you posted a screenshot with exactly that option highlighted.

If I alternate exercises (25 crawl, 25 backstroke, 50 butterfly) on a 50m pool? It will just be enough to be able to put 25m on a 50m pool, right?

Otherwise there is indeed the backstroke but not the crawl. There is the backstroke, breaststroke and butterfly

There is the backstroke, breaststroke and butterfly

Crawl is there too - it is called Free (as in Freestyle), translated as "Libre" (in French the official name of the disciplin is Nage Libre)

Ok thank you and for 25m exercises in a 50m pool, any idea?

See my first answer