The 500 Internal Server Error is a very general HTTP status code that means something has gone wrong on the web site’s server, but the server could not be more specific on what the exact problem is.
Most probably you have a fresh new hosting server. This error is because you have to upgrade your memory size and upload filesize memory in order to allow your server to upload and install properly themes and plugins.
1. You can edit the .htaccess or php.ini files and add few rules to fix this settings. (If you do not have these files, create them in the root of your wordpress)
php_value memory_limit 30M
php_value post_max_size 100M
php_value upload_max_filesize 30M
Use all 3 in .htaccess after everything at last line. php_value post_max_size must be more than than the remaining two.
What to do to correct this is create a file called php.ini and save it in the same location as your .htaccess file and enter the following code instead:
upload_max_filesize = “250M”
post_max_size = “250M”
If after this, still have issues with your server, then go to step 2: