Hi,
We are using Atlassian Bitbucket v5.4.1 and the help text found under 'More information' is wrong. Our main branch is called 'Stable' (protected) and that is possibly the reason for the issue.
When a merge conflict is found the following text is shown:
This pull request can't be merged.
You will need to resolve conflicts to be able to merge. More information.
Clicking 'More information' shows this:

Hope you can help ...
/Jesper