Add plugin search doesnt work

Can you please your browsers console window and determine if there are any sort of Javascript / Ajax errors if any.
I believe you might be facing this issue, most probably because of the following reasons-
1. Either there is some JS error left in your theme code files
2. Or any third party plugin installed has left some JS errors
Due to either of the errors, ajax functionality somehow might not be working in your application, thus you are facing this issue.

Please confirm if you are facing or noticing JS errors in the console. And try to address those issues to make Plugin search working.

yes there was a error on console.i fix it from function.php and error not showed but problem is continue?

In that case could you please check if admin-ajax is functioning properly or not.

Try disabling any third party extension if possible & check if it is caused by any extension or so.

i just checked plugins but there is no changing.i dont know how to check admin-ajax, can you tell me? also it continue different browsers

Well, a simple test to check admin-ajax whether working or not is by checking search functionality on other sections such as Pages or Posts.
If it works then the problem would be specific to Plugins page due to some of the other conflicts or edits in core files.
Or else if it does not work then you will have to enable debug mode to true, trace error contents through log file determine the cause of the issue.

ok