If you are really trying to put more than 3000 issues onto a roadmap in Jira Cloud (or more than 300 epics), you're probably hitting a documented limitation.
The troubleshooting the roadmap link provided in the error offers some suggestions (and states these limits). The workarounds provided are mostly about fine-tuning the filter used for the roadmap, to exclude things you don't need (like issues that are Done, excessive detail like subtasks, issues not modified in the last year, etc).
You can copy/paste your roadmap filter JQL into an "Issues Search" to be able to browse/sort/review all the issues collected by your filter. Those results may also be limited (e.g. max results of 1000), but by using creative sorting you can use different perspectives on your Big List Of Issues to try to identify categories of issues that you might want to exclude from your filter.
One other option is to split your One Big Roadmap into two or more separate roadmaps that focus on smaller sets of issues (e.g. different product sets, 2021 vs 2022 releases, etc). Just a workaround option, probably not your first choice, but it may allow you to get some roadmapping done.
As a last resort, you could reach out to Atlassian Support to see if they might be able to increase that limit for your instance. They might not be able to do that. But if they can, be aware that there will likely be significant performance impact.
Hope that helps. Would be nice to hear back what you find that works for you. Best wishes!
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