In which folder is the server certificate?
The one created by Let's Encrypt.

In the folder /usr/local/psa/var/modules/letsencrypt/etc/live/[domain.tld] there are only the domains, nothing about the server.

In the folder /usr/local/psa/var/certificates/ there are files with random name.

Convert certificates from extension.pem into .crt and .key

Install OpenSSL if is not opresent in your system

To convert an SSL private key from  to , you can use OpenSSL. Here's how:

Install OpenSSL (if not already installed):

sudo apt-get install openssl

Run the Conversion Command: Use the following command to convert the .pem  file to .key

openssl rsa -in privkey.pem -out privkey.key

do the same for certicate

 openssl x509 -outform der -in certificate.pem -out certificate.crt