Why does this sentence "Happy YELLOW DAY Happy YELLOW DAYHappy YELLOW DAYHappy" ?

The rose highlighting usually indicates a missing font.

Try Type menu > Find Font to possibly repair it.

Hello @Hussein ali CRE,

Thanks for reaching out. Could you share more details, like the version of the OS/Illustrator, and if this happens when copying from all programs, so I can better assist you?

Also, try rebooting your computer in Safe Mode (Windows / macOS) and then running Illustrator to see if it helps.

Looking forward to hearing from you.

Anubhav