There are three possible reasons.
(I) You have used Auto-Match today.
Each user can use Auto-Match once per day. If you have used it today, you need to wait for 24 hours in order to use it again.
(II) You have reached your daily new friends limit (12 per day)
This count includes both incoming and outgoing letters from Manually Explore and Auto-Match. If you reached this limit, you need to wait for 24 hours to add new friends again.
(III) You have reached the maximum number of friends (100 per account)
Each user can have a maximum of 100 active friends on Slowly, provided that Removed Users do not count. To add additional new friends, you need to remove some friends from your Pen Friends list in order to free up the quota.
After freeing up the quota, you need to manually turn on the "Accepting New Friends and Auto-match" by these steps.
- Go to My Profile (hamburger tab, bottom menu)
- Scroll down to Matching Preferences
- Turn on Accepting New Friends and Accepting Auto-match