Today I have faced this problem as well.

I found this answer on another, closed topic:

"Certain cards were locked as they were not intended to be out for the moment.

They'll be released in future patches (Just as some were this week)"

I didnt know it. I changed my team to try another lineup so i removed my lvl6 and lvl7 cards from my Anbu Itachi. But now, i can give back to him only 3 cards becouse of my Five Kage Summit Temari says: "The unit is not yet available, cannot be used to assist".

This is very big problem to me becouse now i cant use his bond skill and i dont have resources to level up another card to lvl6...

I understand to they made mistakes and released a lot of assist cards what was not intended, but this solution is very bad... They should find something else.