I would think there could be any of a number of causes. You would need to
look at:

What version of Excel and any other VB programs was on each machine
If it had been properly installed, including being registered
What libraries were shown as references in the VB editor on each machine
If the application was distributed as compiled, whether the runtime libraries
on the target machines were compatible (Applicable only if it was not
distributed as an excel file)
If the registry entries were complete
etc.

If none of those enquiries produced anything useful you could try entering the
VBA code  on the target machines and see if it compiled there without errors.

You could look at the help files on the target machines and see if they make
any comments about the strings library.

I hope you understand that because we do not know what you are doing and what
the setup is on your PC and the target PCs it is not practical for us to do
this sort of troubleshooting.