This is actually an error message from base R rather than from limma. The error says that Group contains 0 or 1 levels, so your assumption that it contains 'Low_Grade'' and ''High_Grade" cannot be true.
to see what the levels of Group actually are.
This is just a wild guess, but one possibility is that you have simply forgotten to capitalize Group, i.e., you used
Group <- factor(sample_info$group)
Group <- factor(sample_info$Group)
would have been correct. You can type names(sample_info) to confirm which name is correct.
In general, it would much quicker to you to trouble-shoot simple errors like this at your end rather than asking for help on a public forum. If Group gives an error when input to a standard function like model.matrix, then have a look at what Group contains. The reason for the error should become quickly apparent.