I’m assuming it’s not “mydomain.com”. - for random reasons (like you left it in part of your response but deleted it from most) lets assume it’s something like shopina.us
For me your domain gives a “HTTP/1.1 403 Forbidden” error, even on the domain itself. I’m assuming you are blocking access to everything other than your own IP for testing ( or something like that ) - hence Let’s Encrypt can not reach your domain to verify it.
and the files are uploaded and access them from the link given by sslforfree.com to verify my domain ownership and got the characters but when i press the button download your certificate it gave the error below
I’d start by checking your access logs for attempts to reach .well-known/acme-challenge/xxd7agat3eSzmEvpGoNRWaPjGRQllfAx7igST_2xpqI and see why they were blocked.
It may be in the error logs, yes. Alternatively there may be information in the normal access log - it depends how things are set up on your shared hosting.