From my experience with GitLab API, there will be required some sort of handling before running AcceptMergeRequest.

There might occur some race condition, when you create a merge request, and it's pipeline is not scheduled yet - running AcceptMergeRequest immediately will just throw some random errors from API.

EDIT: Maybe some polling with a small timeout watching for a head pipeline?

Created a pull request with possible solution at this PR.

Thanks for the PR and for testing.
You are right.
I found a very simple Gitlab repository that I can use for testing and testing this PR returns

Target branch:	"master"
ERROR: action stage:	"set auto-merge on GitLab mergerequest: PUT https://gitlab.com/api/v4/projects/olblak%2Fdocker-examples/merge_requests/1/merge: 401 {message: 401 Unauthorized}"

Your code is probably in the right direction, I'll test it later today

After additional tests, it appears that I needed an access token with at least the maintainer permission and scope set to API

I think the retry you are mentioning is already handled by this https://github.com/olblak/updatecli/blob/eac7e4b42a6aa3ab21e96c96a9c2ef1b5c3d8f85/pkg/plugins/resources/gitlab/mergerequest/create.go#L101

Which is the code snippet heavily inspired from https://pkg.go.dev/gitlab.com/gitlab-org/api/client-go#example-WithRequestRetry-CreateMergeRequestAndSetAutoMerge

Yup, this might be sufficient, needs testing simulating "slow" environment

I am going to merge this PR. We can always improve in the future