So, I've been able to embryo a few mares, but when I try with one particular match I get the error "Error: on insert". It is only this mare and stallion, other matches have been working.

Already been posted in bugs. If you purchased an embryo, it currently doesn't work

Try it now.

It was not a purchased embryo, I own this mare and purchased the straw.

I tried it again and got the same error.

Update: Still not working. It's a Khitla straw and my mare Horse Eden - Online Horse Game. The straw and mare show up on the dropdown lists, but I get the error code when attempting to create the embryo. I tried the mare with a Indalo Kasui straw as well and I still could not make an embryo off her.

Eventually I gave up and bred her.