I cannot add the 6th member as my family member

i have myself and 4 other family members i can add 1 more. to send by text it allows me to put in ph.# but send is grayed out, try emailing it same thing. I can compose email but send icon is grayed out.

Are you trying to add 6 plus yourself or 5 plus yourself? See the following from Family Sharing - Apple

Family Sharing lets you and up to five other family members share access ...