You can use
--allow-unrelated-histories to force the merge to happen.
The reason behind this is that default behavior has changed since Git 2.9:
“git merge” used to allow merging two branches that have no common
base by default, which led to a brand new history of an existing
project created and then get pulled by an unsuspecting maintainer,
which allowed an unnecessary parallel history merged into the
existing project. The command has been taught not to allow this by
default, with an escape hatch
to be used in a rare event that merges histories of two projects
that started their lives independently.
See the Git release changelog for more information.