Can't use the "Save As" option


Hi, hopefully someone can help me.
I use Smart Office on my Samsung Galaxy phone (several models including S6 & S8)
and I have recently had a problem trying save xls documents.
When I attempt to use the “Save As” option to enable me to make changes to the document, assign it a different number and still retain the original I get this message;
Error Saving Document
I can save the changed document as a PDF but obviously this doesn’t allow me to make further changes should I need to.