error": {
"code": "Plumsail.Actions.Api.DTO.Exceptions.DataValidationException",
"message": "The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters. ",
Try to use base64() expression in the Documents action field to process the file content data you want to use. I don't know which exactly action you use so I got the "Compose" action for example (don't pay attention to other actions).
You need to select the base64 function in the Documents action field. I don't see the Documents action in the screenshot, but I suspect it's in the Apply to each section, isn't it? Please share a screenshot in Edit mode.
Actually document content file should have raw content of the source .xlsx template file. usually it's extracted from other connectors like: SharePoint, OneDrive, etc
I'd recommend you placing the xlsx template to OneDrive, use Get file OneDrive action and select the Get file result in Plumsail action Document content field