There may be more detailed issue specifically related to your situation, but this thread:
https://discussions.scouting.org/t/who-has-permission-to-assign-merit-badge-counselors-now/510295
has a fairly extensive discussion that, I think, concludes that there is a system issue right now that is preventing otherwise proper permissions from working with regard to connecting MBCs to scouts.
Two workarounds are to have the Unit Key 3 (or Key 3 Delegates they designate in my.scouting) make those connections in SB+, or to have the scout/scout’s parents provide the information to the MBC, and have the MBC initiate the connection from their end in SB+. I posted some screenshots for “Option 2” in that thread here:
https://discussions.scouting.org/t/who-has-permission-to-assign-merit-badge-counselors-now/510295/39
