More explicit message: spec_to_metacore("Metadata/ADaM_Specifications_TEST.xlsx")

[cpiraux]

After reading this message, I got the impression that I may not be able to utilize the metacore package with my input file. It would be helpful to provide some alternatives, such as following the p21 template for the Excel file.
I was also wondering if it would be possible to update doc (doc <- read_all_sheets("xxx.xlsx")) to concatenate all sheets with the same structure in one sheet. Alternatively, would it be possible to have a vector of sheets in the function argument sheet = c()?