Choice of Scheme when formatting USB flash drive in macOS
In virtually all cases, you want GUID Partition Map. Only choose a different scheme if you have specific needs that require that scheme. GUID Partition Map provides the best compatibility and is the most current scheme.

Without knowing what the error is, I can't say why you're unable to erase the disk. The top level item is the disk itself, whereas the lower level items are partitions. When erasing a specific partition, you're only changing that partition, not the whole disk. Since the scheme is for the entire disk and not one partition, it doesn't make sense to have the scheme show for a partition.