SEO, website optimisation and crawability issues

SEO
I need some help to perform keyword research
- Perform niche keyword research
- Identify keyword clusters and sections where to use them
- Provide input on keyword strategy and how to approach the task

Optimize site-speed
checking the site on “PAGE SPEED INSIGHTS” (current result: 7), creating AMP pages and implementing a DNS.
—> OPTIMOL-plugin already implemented
—> increase “green” result on “page speed insights”
https://developers.google.com/speed/pagespeed/insights/?url=https%3A%2F%2Fleben-arbeiten.com%2F

Trouble-shot site audit
Site is currently blocked from the Site Audit crawler
(Want to allow SEMrush Site Audit bot (SemrushBot-SA) to crawl my site)
—> check the site's IP access rules and whitelist the following IPs performing Site audit:
https://www.semrush.com/kb/681-site-audit-troubleshooting#whitelist
IPs to be whitelisted are:
46.229.173.68
46.229.173.67
46.229.173.66

Current response from my server when trying to perform a CURL command for my domain:
curl -k -i -sS -L --proto-redir -all,http,https --max-time 5 -A 'Mozilla/5.0 (compatible; SemrushBot-SA/0.97;+http://www.semrush.com/bot.html)' -H 'Accept-Encoding: gzip, deflate' -H'Accept: */*' --compressed http://leben-arbeiten.com
HTTP/1.1 406 Not Acceptable
Date: Wed, 03 Jun 2020 14:18:42 GMT
Server: Apache
Content-Length: 226
Content-Type: text/html; charset=iso-8859-1
Not Acceptable!Not Acceptable!An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.