Must probably you are using Free Test API key here. Free Test API key only allow single font conversion request which might have been used. To upload more fonts, you need to get premium key. It supports multiple domain and multiple font conversion. For other cases, please check below ↓
Uploaded font once, deleted it and re-uploading again
It still counts as used as Test API key is for single font conversion only. If you still need to test, you can re-generate another Free API key or go premium to limit the limitation.
First time got error during upload, and now says already been used.
Once the font reaches to our convertor server, converts it and sends back. It is counted as used. So please kindly re-generate another API key and try again. For error during font upload, please check Font upload section below.
Got premium key, still unable to upload more than 1 font.
Please make sure, you have removed the Test API key and using premium one.