It’s the first day of the season. In the previous season I offered contracts to players in the detection module. But they have not arrived yet. Is this a bug?

Forgive me, seems like I received the detected players

Hello,

Go to Youth Academy > Detection > tab contracts.
The juniors that comes in your club have a "green contract icon", those who goes in another club have a "grey contract icon".
Those icons are links too.

PS : don't forget to organize friendly games for your junior squad, to help them to reach 100% faster.

It can take a while at the start of a new season, especially if you were prospecting in a country with an english name starting with last letters, like Wales, Zambia or countries like that.

Hello, thank you.
How can you promote multiple players from the youth academy all at once?

No. Just one at a time.