Can't open a old (4-5 years) PSB file? It needs an older Version but Adobe don't allow older Version

I have an old PSB-file with a Composing for a customer of mine. He wants to update this composing, but I can't open the PSB-files?

"The operation could not be performed,
Because this is not a valid Photoshop document. "

and

"Failed to open “XXXX.psb” because this file is not compatible with this version of Photoshop."

It seems I need an older Version of Photoshop, unfortunately Adobe Cloud don't let me install Versions older than Version 21.2.11 .... But I created the original file with an Adobe Abo?

So, files are just gone???

What is this...