From the StringFunctions documentation there is no Common nor MsgBox function.
What B4A line gives you the error?
Probably it needs a new compilation with B4A 6.80.
Unfortunately, the author, margret, seems to be no more active in the forum.
I suppose that only she has the source code.
In the first time StringFunctions was a Code module, and then it was compiled into a library.
Now, my pesronal opinion.
I have never used StringFunctions, why?
In my opinion, StringFunctions was wriiten to add VB style methods to B4A.
When I use a language, I use whenever I can the native methods instead of workarounds adapting previous habits to the 'new' language.
But, as already said, that's just my opinion.