can't send calendar activities to my garmin 645 music

What do you mean by "calendar activities"? Normally you sync Activities from the watch to the Garmin Connect, and not oppositely. Why do you want to send the to the watch?

Its about training plan which is write in my calendar. It used to be an icon „send to device”. Now its gone and i cant send training plan from calendar to watch...

Try making some minor change to the plan, and the "send to device" button should reappear

Thx i will try