Specify and existing value

Hi experts,

I use IDES and created company code 9510, I want to create new GL account, when I use T-code FS00, It shows COA of company code 1000, when I change COA to my company code, then system shows (Information error) that "Specify an existing value".

I don't know what does it mean?

Kindly help me to resolve it.

Thanks