Not in FMP6. I'm actually surprised this is the only issue you see...I had nightmares over FMP8 on Windows 7.
Starting in FMP8 there is a SetVariable step that, combined with SetField, gave a better way to "copy and paste"...ie. SetVariable and SetField...that did not involve the Computer clipboard. This is now the best practice (FMP8 and later).
In FM6, declare a field with global storage. Set the global field to the value instead of copying it. Set your target field from the global field instead of pasting.
The most obvious way in which copy can fail is that the field to be copied from must be on the current layout. But in Windows 7 and 8, there appears to be another issue or two involved.
There is an ongoing discussion of why copy may fail here: Clipboard not working
but i cant set and get global field between different database ... or not
Yes you can. Did it all the time way back then. Define a relationship linking the two tables/files and then you can reference the global field in either file.