If now helped quite a few people with this issues and this is what has worked so far. I made a detailed post several days ago with step by step instructions if you want to search for it.
Here are the basics. All the people to date have not had Apple Cash enabled for the new co-owner/applicant. This has proven harder to do for some.
1) Make sure the iOS is 14.6 or higher and install any updates.
2) Restart iPhone
3) Log out of spouses iCloud account
4) Restart iPhone & log into iCloud with spouses Apple ID and password
5) add Apple Cash to Wallet app
6) restart iPhone.
7) Restart you iPhone
8) Go to your wallet app and invite spouse.
If still grayed out, post back.
I hope this helps and good luck!