I would suggest you to please add debug logs while performing this operation as there is an error happening in background whic could be validation problems and other reasons which cannot be identified without logs.
Kindly let me know if it helps you and close your query by marking it as solved so that it can help others in the future. It will help to keep this community clean.
Thanks
Greetings to you!
Please refer to the below Knowledge Article which might help you further with the above issue.
Error when a lead is converted
Knowledge Article Number 000318937
https://help.salesforce.com/articleView?id=000318937&type=1&mode=1 (https://help.salesforce.com/articleView?id=000318937&type=1&mode=1)
I hope it helps you.
Kindly let me know if it helps you and close your query by marking it as solved so that it can help others in the future. It will help to keep this community clean.
Thanks and Regards,
Khan Anas