bizarre plus signs

The + sign indicates that the duration of the measure is longer than that indicated by the time signature. Similarly a - sign indicates a duration shorter than indicated by the time signature. Sometimes you want this, often you don't. It often occurs when you import a pdf. That process involves optical music recognition which, if the original pdf is not clear enough, may misread note or rest durations - for example misread a triplet as three eighth notes or a sixteenth as an eighth - and therefore get the total duration wrong.

What to do about it if you don't actually want that longer duration? You need to inspect the measure to see what has been misread and then edit or replace it to give the correct duration. Such inspection and editing can be complicated and time consuming. It is often simpler and quicker to transcribe a pdf manually rather than trying to fix the mess that results from an import of a pdf.

If you attach the score here someone with more patience than I have may be able to give guidance on whether your irregular measures can be easily corrected or whether it is better to give up and start again.

"The measure is in the middle of the song so I can't just delete the measure and they won't allow the correct rhythm to be written."

You can "just delete the measure": select the measure and press Ctrl+Delete.

However, I would advise that you first insert a new measure in the vicinity and input the correct notation there (so you can compare with the incorrect measure). Only then use Ctrl+Delete to delete the incorrect measure.

It may be possible to "fix" the incorrect measure. However, in my experience, it's usually easier and quicker to just re-input a measure and delete the incorrect one.