Forecast Hierarchy

I'm looking at implementing a Forecasting Hirearchy however, I've noticed some of our Roles do not have the option to "Assign Manager":I'm most interested in those Roles under "Sales" - why is it that Corporate and Wholesale do not have the "Assign Manager" option but Mid Market/SMB Team and Enterprise & New Logo Team do..?

I've been googling and searching on here, but it's not clear to me what the issue is.

Please help!

Liz