JMETER - response assertion -> Response was null - jmeter

Question

I have checked several options how to validate response and it still not works in my case.
As you can see (Debug sampler), for last_name regex works and I receive some values, but if I want to reuse this response as "Response assertion", test failed, because I receive "Response was null".
JSON Extractor for last_name
Response assertion
Test failed
Also, I would like to count objects in "data", so I used http://jsonpath.herokuapp.com/?path=$..book[0,1] to make sure, the JSON file is correct. But "Debug sampler" shows me nothing :( so test failed again
Count of objects
Result
Any idea how to solve this problem please ?
JSON:
{
"page": 2,
"per_page": 6,
"total": 12,
"total_pages": 2,
"data": [
{
"id": 7,
"email": "michael.lawson#reqres.in",
"first_name": "Michael",
"last_name": "Lawson",
"avatar": "https://reqres.in/img/faces/7-image.jpg"
},
{
"id": 8,
"email": "lindsay.ferguson#reqres.in",
"first_name": "Lindsay",
"last_name": "Ferguson",
"avatar": "https://reqres.in/img/faces/8-image.jpg"
},
{
"id": 9,
"email": "tobias.funke#reqres.in",
"first_name": "Tobias",
"last_name": "Funke",
"avatar": "https://reqres.in/img/faces/9-image.jpg"
},
{
"id": 10,
"email": "byron.fields#reqres.in",
"first_name": "Byron",
"last_name": "Fields",
"avatar": "https://reqres.in/img/faces/10-image.jpg"
},
{
"id": 11,
"email": "george.edwards#reqres.in",
"first_name": "George",
"last_name": "Edwards",
"avatar": "https://reqres.in/img/faces/11-image.jpg"
},
{
"id": 12,
"email": "rachel.howell#reqres.in",
"first_name": "Rachel",
"last_name": "Howell",
"avatar": "https://reqres.in/img/faces/12-image.jpg"
}
],

Answer 1

You need to tick Compute concatenation var in JSON Extractor, otherwise you will get 2 separate JMeter Variables:
lastName_1=Lawson
lastName_2=Ferguson
In addition to point 1 you need to use lastName_ALL in the Response Assertion
You need to switch from Matches to Substring in "Pattern Matching Rules" of the Response assertion
Demo:
More information: How to Use the JSON Extractor For Testing