Many Cloudflare users report issues when trying to import a free Let’s Encrypt SSL certificate into their domain, on a Cloudflare free account. Try the following steps:

Login into CF and select domain you want to work with.
Select “Crypto” top menu option
Under SSL select – Full
Set Always use HTTPS to ON
On HSTS section – Enable HSTS
Max-Age: 3 months
Include subdomains: Off (change as you wish – read up on it)
Preload: Off
Set to Minimum TLS Version to TLS 1.2
Opportunistic Encryption: ON
TLS 1.3: ON
Automatic HTTPS Rewrites: On
Disable Universal WordPress SSL by doing this you are no longer using CF SSL certs and use only Certs served by your server.

I think this is an important bit to add since many LE / CF users have issues with the process outlined in the post.

Would that work?