I have this problem it happened randomly synthesis used to work fine but it telling me mods are referring to fish.esm as master and it's missing or has different references I reinstalled everything including fish.esm from creation club same problem .

[98hocinemesrouk]

ingModException ccBGSSSE001-Fish.esm: A referenced mod was not present on the load order being sorted against: ccBGSSSE001-Fish.esm. This mod was referenced by MajorRecord: 05BF21:Skyrim.esm at Mutagen.Bethesda.Plugins.Binary.Translations.ModHeaderWriteLogic.SortMasters(List1 modKeys) at Mutagen.Bethesda.Plugins.Binary.Translations.ModHeaderWriteLogic.ConstructWriteMasters(IModGetter mod) at Mutagen.Bethesda.Plugins.Binary.Translations.ModHeaderWriteLogic.PostProcessAdjustments(MutagenWriter writer, IModGetter mod, IModHeaderCommon modHeader) at Mutagen.Bethesda.Plugins.Binary.Translations.ModHeaderWriteLogic.WriteHeader(BinaryWriteParameters param, MutagenWriter writer, IModGetter mod, IModHeaderCommon modHeader, ModKey modKey) at Mutagen.Bethesda.Skyrim.SkyrimModCommon.WriteParallel(ISkyrimModGetter item, Stream stream, BinaryWriteParameters param, ParallelWriteParameters parallelParam, ModKey modKey) at Mutagen.Bethesda.Skyrim.SkyrimModMixIn.WriteToBinaryParallel(ISkyrimModGetter item, String path, BinaryWriteParameters param, ParallelWriteParameters parallelParam, IFileSystem fileSystem) at Mutagen.Bethesda.Skyrim.SkyrimMod.Mutagen.Bethesda.Plugins.Records.IModGetter.WriteToBinaryParallel(FilePath path, BinaryWriteParameters param, IFileSystem fileSystem, ParallelWriteParameters parallelWriteParams) at Mutagen.Bethesda.Synthesis.SynthesisPipeline.Run(RunSynthesisMutagenPatcher args, Nullable1 exportKey, IFileSystem fileSystem) in D:\a\Synthesis\Synthesis\Mutagen.Bethesda.Synthesis\SynthesisPipeline.cs:line 566 at Mutagen.Bethesda.Synthesis.SynthesisPipeline.Run(RunSynthesisMutagenPatcher args, IFileSystem fileSystem) in D:\a\Synthesis\Synthesis\Mutagen.Bethesda.Synthesis\SynthesisPipeline.cs:line 456 at Mutagen.Bethesda.Synthesis.SynthesisPipeline.<>c__DisplayClass36_0.<b__1>d.MoveNext() in D:\a\Synthesis\Synthesis\Mutagen.Bethesda.Synthesis\SynthesisPipeline.cs:line 419

[Noggog]

I think this has been fixed as a bug