won’t let me set up family sharing

Firstly, each 'Family Member' have to have their own Apple ID and best to be already listed as your contacts. Family Sharing can only (that I know of) be set up by the administrator of the device.

For Mac's, generally:

1. Go to Preference
2. Click iCloud
3. Click Manage Family
4. Click Add Family Member... (or the "+" sign at the bottom of the window that list Family Sharing Members)
5. Select "Enter Family Member Name" or "Create Apple ID for a Child" (either type their name, email address, etc. or follow the steps to set up an Apple ID for a child
6. Click Continue
7. Enter the Administrator Password
8. Click Continue
9. Enter the 3 digit CVV found on the back of the credit card saved on the Administrator's iTunes/Apple ID account
10. Click Continue

An invite will be sent to the person you're wanting to add. They just need to accept it and then the accounts will be linked up.

For iPhone's and iPad's, generally:

1. Under Settings; select on where your name and picture is at the top to bring up the Apple ID info page
2. Select Family Sharing
3. Select Add Family Member
4. Select "Invite via iMessage" (an iMessage will be sent to the person with a link for the setup process) or "Invite in Person" (the person will be asked to enter their Apple ID and Password) or "Create a Child Account" (you will basically need to go through setting up an account)

Just like with a Mac, the Administrator will need to provide the credit card CVV for security reasons, etc. to finish up the linking accounts process.

Follow these steps here: Set up Family Sharing - Apple Support