x509: certificate signed by unknown authority

i follow this commands from [real-python](https://realpython.com/python-microservices-grpc)

`
openssl req -x509 -nodes -newkey rsa:4096 -keyout ca.key -out ca.pem -subj /O=me

openssl req -nodes -newkey rsa:4096 -keyout server.key -out server.csr -subj /CN=recommendations

openssl x509 -req -in server.csr -CA ca.pem -CAkey ca.key -set_serial 1 -out server.pem
`

and i cant connect to the server

grpcurl -cacert=ca.pem -d='{"id":""}' user-service:443 app.xis.User/GetUser
i even tried

grpcurl -cacert=ca.pem -cert=./user/server.pem -key=./user/server.key -d='{"id":""}' user-service:443 app.xis.User/GetUser

idk what's happening, anyone could help me?