Couldn't join realm: Insufficient permissions to join the domain

Solution 1:

I had this problem on a home domain set up using Ubuntu 20.04 (both server with domain controller on samba and all domain members). The solution turned out to be very simple. Before You can succesfully join You need to modify the /etc/hosts file to map the ip address to the domain controller host(s). You can remove these entries after succesfully joining the domain, as then Your new domain member will use the dns in the domain, but before that happens I think it's kinda lost. To me these entries looked similar to this:

192.168.1.10 primarydc.mydomain.com primarydc
192.168.1.10 primarykrb.mydomain.com primarykrb