The User could have disabled the option to accept Friend Requests. (Profile > Smaller Profile > Edit Profile and Privacy > Privacy).
If this is not the case, please try the following:
To clear the Friend Request:
1. Select your Friend's Profile on the app > Tap the 3DOTS to the right of the message button > Tap 'Block Friendship’ > Select 'yes' to confirm.
2. Navigate away from the profile, then return to the user's profile to tap on the 3DOTS again, this time tap Un-block, and Yes to confirm.
This will clear the invite and you should see the 'Friend' button again. Tap on it to send an invite to your friend.