The User could have disabled the option to accept Friend Requests. Select the Profile icon > Small Profile icon (located top left) > Edit Profile and 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 and tap Un-block, and Yes to confirm.
This will clear the invite and you should see the 'Friend' button again.