I don't see a chain file:
openssl s_client -connect squid-proxy-ssl.ops2.cresta.ai:3128 -showcerts
CONNECTED(00000003)
depth=0 CN = squid-proxy.ops2.cresta.ai
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 CN = squid-proxy.ops2.cresta.ai
verify error:num=21:unable to verify the first certificate
verify return:1
depth=0 CN = squid-proxy.ops2.cresta.ai
verify return:1
---
Certificate chain
0 s:CN = squid-proxy.ops2.cresta.ai
i:C = US, O = Let's Encrypt, CN = R3
a:PKEY: rsaEncryption, 2048 (bit); sigalg: RSA-SHA256
v:NotBefore: Oct 20 14:51:53 2023 GMT; NotAfter: Jan 18 14:51:52 2024 GMT
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
---
Server certificate
subject=CN = squid-proxy.ops2.cresta.ai
issuer=C = US, O = Let's Encrypt, CN = R3
---
No client certificate CA names sent
Peer signing digest: SHA256
Peer signature type: RSA-PSS
Server Temp Key: X25519, 253 bits
---
SSL handshake has read 1872 bytes and written 412 bytes
Verification error: unable to verify the first certificate
---
New, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384
Server public key is 2048 bit
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 21 (unable to verify the first certificate)
---