fatal: Unable to create index.lock File exists

The following error was reported during the execution of the pipeline.

gitlab version:13.8.3

runner version:13.8.0

Preparing the "shell" executor
00:00
Preparing environment
00:00
Running on xxx.net...
Getting source from Git repository
00:00
Fetching changes...
Reinitialized existing Git repository in /home/gitlab-runner/builds/G-tus7wz/0/xxx/.git/
Checking out 403fb1ee as trunk...
fatal: Unable to create '/home/gitlab-runner/builds/G-tus7wz/0/xxx/.git/index.lock': File exists.
Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.
Cleaning up file based variables
00:00
ERROR: Job failed: exit status 1