butter was too soft/warm when incorporating, meringue was to hot when butter was added, sugar wasn’t 248-258oF when adding to the whites, or cream of tartar was ommited.
before I start, I let the butter reach room temperature and cream it well with the flat beater. Then I chill it in a cooler with one ice pack till butter is cold but thickly spreadable.
I whip my meringue till is lukewrm about 95oF to ideally 85oF, then add the butter.
the thin mayonaise consistency you experiencis discussed on the recipe, to fix it, chill the mixture before fully whipped, about 15 to 20 minutes, then finish whipping.