Why one bar on a system and what fix

Originally these bars were one system until I entered the circled note and suddenly this happened.
And I’ve tried to make into a system again but if I select the first note in first bar and last note on second line and chose make into system a mess ensues.

When making edits near the end of the score, Dorico can sometimes get its knickers in a twist concerning edits made in the range over which it tries to cast off the score in a special fashion to avoid the last system having only one or two bars on it. It basically casts off a different number of systems depending on the number of players in the layout at the end of a flow to try to distribute the bars evenly across those systems. Sometimes, in circumstances we have so far found difficult to pin down, it fails to process the entire range. You may find that if you switch to another layout and then back to full score in the same tab, it sorts it out, and failing that, saving, closing and reopening might take care of it.

Well switching to anither layout and back didn’t work nor did save, quit, and reopen.
I deleted the bars and reentered them.

Now I have a last barr which has 8 beats. The piece up to now is in 4/4.
I’d just like to split the bar in two.
Have tried but can’t figure out how.

You managed to delete the barline as well as the break, I guess. You can put the barline back in by selecting the note at the position of the fifth quarter in the bar, then clicking the normal barline in the Bars panel in Write mode, or do Shift+B and type “|” or “single” into the popover.

Even splitting the bar it still does the same crazy thing when entering notes at a particular point.
I gave up, deleted the score, and started over with an xml version.