This looks like an issue I already reported to @Daphne but seems to not have reached the tutorial yet:
There is a naming error with the PexelsService.php file which should instead be named GifService.php same as the class in it.
The error is sadly not very helpful due to some bug in the dependency injection error handling, but should improve in the future with fix: Throw early when a service cannot be found in the app container by juliushaertl 路 Pull Request #38322 路 nextcloud/server 路 GitHub