Why was the church upset with gregor Mendel?

Answer 1

You may be confusing mendel with someone else. His monastery was very supportive of his work when he was being ignored by the scientific community. After he published his work the elected him unanimously to lead the monastery.

Answer 2

The church was upset with Gregor Mendel because his work on genetics and heredity seemed to challenge the idea of creation and the concept of the immutability of species as presented in The Bible. Mendel's discoveries contradicted the prevailing belief in the spontaneous generation of lifeforms, which was a theological issue for the church at that time.