Guidelines for catalog upgrade :
1. Copy your Lr6 catalog file (only the lrcat file) to an internal drive, where available space is more than twice the size of your catalog. if(your catalog is 10 GB, I recommend you to have available disk space upto 25 GB) .
2. Clear any temp files in the directory where the lrcat file is present.
3. Check if your catalog opens in previous version of Lightroom (6.12)
4. If it does then try optimizing it , File -> Optimize catalog.
5. After optimizing. close all applications. Clear any temp files further created in the directory.
6. Disconnect any external drives connected.
7. Try upgrading your catalog with new LR Classic CC.
If you still face issues, please share your catalog with us. (Zip it, upload it to dropbox, share the link over private message), we will try to debug your problem asap.