After weeks of trial and error, I was able to correct the code. I found an article from Microsoft Learn. This article helped me use an alias to get the error from the "Equal sign" and the .RelJob. There was also another issue I found that my Patch syntax was incorrect.

If(CountRows(colRelatedJob) > 0,
    ForAll(colRelatedJob As _Related,
        If(_Related.SaveRecord,
            Patch(ds_tblRelatedJobs,
                    Coalesce(LookUp(ds_tblRelatedJobs, FK_JobNum = txtJob.Value && RelatedJob = _Related.RelJob),Defaults(ds_tblRelatedJobs)),
                {
                    RelatedJob: _Related.RelJob,
                    RelatedGroup: _Related.RelGroup,
                    RelatedPM: _Related.RelPM,
                    RelatedForeman: _Related.RelForeman,
                    RelatedScope: _Related.RelScope,
                    FK_JobNum: txtJob.Value
                }
            )
        )
    )
);