Generally before start deploying container check on Docker Hub (when Docker registry is used) if specific tag for container image supports your device CPU architecture which is ARM (32-bit) by looking at device product page, image should have builds for linux/arm/v6 or linux/arm/v7 architectures, in this case latest tag (latest tag is fetched if not specified for remote image) in unifi-controller tags have builds only for linux/amd64 (x86-64) and linux/arm64 (ARM64) and that is reason of this error, your device architecture is not supported for this image tag.
Some container images can have different tag for some architecture, like older version tag when latest build is made for arhitecture supported by your device, you can try to filter tags by arm or arm32 keywords to find it, example https://hub.docker.com/r/linuxserver/unifi-controller/tags?page=1&name=arm32 and you can see there is arm32v7-7.3.83 latest tag with supported arch (there is also arm32v7-latest and arm32v7-mongoless tags but with wrong arch), if that older version suits you, you can try to add container with remote image
linuxserver/unifi-controller:arm32v7-7.3.83
.