Certs Issue with local installation

1. What is the problem? Be very detailed.
I am performing a local installation on Centos 7. The domain is on internal network and I am using custom certificate for SSL.
So application comes up fine after applying certificate but some functionalities(Specifically "Preview Form") are not working and throwing error as : “unable to verify the first certificate”
When I download and all the whole chain in the .pem file the error changes to
“self signed certificate in certificate chain”.
Need help to fix this issue. Whole site comes up fine only "preview" throws cert error.

2. What app or server are you using and on what device and operating system? Include version numbers.
Its a Linux server Centos 7 OS.

3. What you have you tried to fix the problem?
I have applied 2 certificates but with same results. Since the domain is internal I cant use LetsEncrypt or any other 3rd party tool.

4. What steps can we take to reproduce the problem?
Just click "Preview" button.

5. Anything else we should know or have? If you have a test form or screenshots or logs, attach below.