I currently have a Family SIM plan with 3 sims. If I click on add sim to add another after clicking through I always get this error "There aren't any plans that you can move to at this time.".
You are on a 3 sim family bundle deal. I think you would need to change to custom package which allows up to 5 sims
I'm not sure this is the case, as I added the 3 SIMS several months apart. Additionally, when I click through I am given the option to add 1 or 2 SIMs, suggesting I'm on a 5 SIM deal if anything?
as I said there is a fixed package or you do custom for up to 5 it's your choice why not try online chat and see if you can clarify what you have and how you can add another sim https://www.bt.com/help/home/mobile/#/
Call in, 0800 800 150. Sometimes family sim orders are a bit funny if you are on a proposition that we no longer provide. An agent will be able to tell you what we can offer and get you changed over.