[Plugin: Google XML Sitemaps] Sitemap is not generated-The last run didn't finish!

When I try to generate sitemap with XML-Sitempa_generator I always get these message.

# The last run didn’t finish! Maybe you can raise the memory or time limit for PHP scripts. Learn more
# The last known memory usage of the script was 28.5MB, the limit of your server is 256M.
# The last known execution time of the script was 2.13 seconds, the limit of your server is 500 seconds.
# The script stopped around post number 4593 (+/- 100)

What is wrong? How can I fix these?

Thank you.