Please know that to solve the error we have to follow below steps.

Step-1:  Please see the error screenshot below. if you get error like this then follow the steps.

Step-2;  Login to your server panel and  go to public_html folder  inside the folder you will see all the files & folders of the website. Below you will see  file named php.ini  edit the file. Please see the screenshot below.

Step -3:  Now you have to edit the code   memory_limit = 64M;   change the line of code to  memory_limit = 512M;  and save it

Step-4:  Now  the same line of code you have change inside the admin folder.

Step-5:  Open admin folder here also you will see the same file name php.ini  please see the screenshot below

Step-6: edit the the line of code  memory_limit = 64M;  to  memory_limit = 512M; 

Note:  If the issue did not get solved after changing the line of code please contact your server provider & ask to increase the php memory,  because it is an server related issue.