The following are possibilities:
1. Ensure you are using the Excel interface correctly. Return to the Foundation Training exercises if you are not sure. I have seen an example where a client was getting this error because they used Create Instance with the Enable Events input parameter set to ""True"" (which is text because of the quotations) instead of True.
2. Make sure you are using a version of office that is compatible with your version of Blue Prism. Blue Prism has only been certified for use with Office 2016 in one of the latest releases (check the release notes).