can’t increase a memory limit- 500 error only admin area

Hello,

I can’t increase a memory limit.

I found a 500 error (short time) problem in my WP admin area, when I click a function or the page or save.
If I click backward that it’s working.
I think this problem effect from a memory limit.
So I edit a memory limit of wp-config to 256M completely.
/** Memory Limit */
define(‘WP_MEMORY_LIMIT’, ‘256M’);
define( ‘WP_MAX_MEMORY_LIMIT’, ‘256M’ );

But my site still has a memory limit=64m.
My web hosting details;
-PHP version 7
-Memory limit 516M

How to fix it?
Please advise.

Thank you,
Thananit