Having almost the exact same issue, had upgraded to V7 and want to extend the trial and just keep getting "Error: Trial extend" whenever I try.
Have logged out and back into account.unraid.net multiple times but just keep getting same error.
About the only difference is my trial expired a few months ago, I haven't done anything about it till now as been looking after a sick family member so evaluating unraid took a back seat but I can get back into it now.
Apart from logging out of account.unraid.net and re-logging in, any other things I can try?