Why was your LinkedIn invitation not sent? Usually because of your own account, not the person you tried to reach. LinkedIn's help page on sending invitations says a primary email address is mandatory to send them. LinkedIn also stops an account from sending invitations for a while after many go out in a short time, or after many are ignored, left pending, or marked as spam.
Check your account first
Start from what LinkedIn showed you, not from the person.
Send is greyed out on every invitation. Your primary email address is unconfirmed, newly added, or bounced. Confirm it in your LinkedIn settings.
A notice that you reached the invitation limit. Your account is restricted from sending invitations for now. Every plan has the limit, and LinkedIn will not lift it on request.
A box asking for the person's email address. They accept invitations only from people who know it. Enter it only if you already do, and never guess.
A box asking how you know them. LinkedIn may want a message first for someone you have not met, which can mean an InMail.
The first two stop every invitation you try, so fix those first. The last two are about one person.
How to read the decoder
For this symptom the decoder says to try later. Later means once your email address is confirmed, or once LinkedIn's wait for a restriction is over. It never means pressing Send again now. Pressing again, or trying from another browser, neither confirms an email address nor lifts a restriction.
A missing Connect button is a different case, and why there is no connect button on LinkedIn covers it.
What to do next
Confirm your primary email address, then send one invitation, once.
If LinkedIn shows a limit notice, stop sending invitations and wait as it says. Withdraw old invitations one at a time, and when you start again, send fewer, to people who would recognize your name. The LinkedIn invitation limit has the waits.
If the notice is about your whole account, read the causes of a restricted LinkedIn account before anything else.
In a Oneprofile sequence, a connection request that cannot be sent is skipped, and the contact's history names the reason. Put a follow or an email after it, not a second request.