why my adobe illustrator often failed to copy and paste an objects?

Does the copy and paste work when you use the commands from the menu? Edit > Copy/Paste/Cut?

General issues with the clipboard are unfortunately widespread. YOu will find out when you sue Safe Mode, becasue that disables all third party extensions that might interfere. It's also crucial that you ppen only Illustrator and particularly not a browser, because some browsers are known to interfere with the clipboard badly.

Hello @Joni5E0B,

We understand that encountering technical issues can be frustrating. I hope the suggestions shared by Mylenium, Met1, and Rene helped resolve the problem. If not, kindly try rebooting your computer in Safe Mode (Windows / macOS) and relaunching Illustrator and check if it helps. We appreciate your time and effort in helping us resolve this issue.

Looking forward to your response.

Thanks,

Anubhav

Try a prefs reset.

Try running Illy with a safe restart and don't run anything else - I think someone was having issues with Word or Chrome; search the forum, it was asked recently.

Do you otherwise have problems with your modifier keys that they don’t work properly? Could also be an issue with the keyboard. 

Without any system info and specifics on when the operations fail we can't tell you much. Otherwise simply refer to this:

Install previous versions of your Creative Cloud apps

Mylenium