There are a few reasons this could happen. You may have disabled Friend Requests. To view this setting:
1. Select the Profile icon located in the upper left.
2. Click Settings (gear icon) in the upper right corner.
3. Go to Privacy then toggle on the Friend Request icon.
If they haven't disabled Friend Requests, please try the following:
To clear the Friend Request:
1. Select your Friend's Profile on the app.
2. Tap the 3 dots to the right of the message button.
3. Select 'Block this person".
4. Select 'Yes' to confirm.
Refresh the app; log out, then log back into the app.
1. To log out: Tap the Profile icon located in the upper right.
2. Select Settings and Scroll down.
3. Click 'Sign out.'
4. Return to the User's profile and tap Unblock.
5. Select 'Yes' to confirm.
This will clear the invite and you should see the 'Friend' button again.