First map crawl to refinable and then start incremental or full crawl.
Test this solution
https://afrait.com/blog/display-all-checked-out-files-from-current-user-en/
In query you set Checkoutuser isnotblank and that content type
EDIT
I found it :)
https://ktnnsharepoint.wordpress.com/2017/03/01/search-checkedout-files/
You need to place that managed property in query like this to get all documents that are checkout.
CheckoutUserOWSUSER:a* OR CheckoutUserOWSUSER:b* OR CheckoutUserOWSUSER:c* OR CheckoutUserOWSUSER:d* OR CheckoutUserOWSUSER:e* OR CheckoutUserOWSUSER:f* OR CheckoutUserOWSUSER:g* OR CheckoutUserOWSUSER:h* OR CheckoutUserOWSUSER:i* OR CheckoutUserOWSUSER:j* OR CheckoutUserOWSUSER:k* OR CheckoutUserOWSUSER:l* OR CheckoutUserOWSUSER:m* OR CheckoutUserOWSUSER:n* OR CheckoutUserOWSUSER:o* OR CheckoutUserOWSUSER:p* OR CheckoutUserOWSUSER:q* OR CheckoutUserOWSUSER:r* OR CheckoutUserOWSUSER:s* OR CheckoutUserOWSUSER:t* OR CheckoutUserOWSUSER:u* OR CheckoutUserOWSUSER:v* OR CheckoutUserOWSUSER:w* OR CheckoutUserOWSUSER:x* OR CheckoutUserOWSUSER:y* OR CheckoutUserOWSUSER:z* OR CheckoutUserOWSUSER:1* OR CheckoutUserOWSUSER:2* OR CheckoutUserOWSUSER:3* OR CheckoutUserOWSUSER:4* OR CheckoutUserOWSUSER:5* OR CheckoutUserOWSUSER:6* OR CheckoutUserOWSUSER:7* OR CheckoutUserOWSUSER:8* OR CheckoutUserOWSUSER:9* OR CheckoutUserOWSUSER:0*