Trying to find accident or how is called...

Okay, so the two measures actually have quite different issues going on. Let's begin with the easy one: measure 8, immediately before the start-repeat sign.

You have Voice 1 (dotted half note) already entered, so all you need is to enter Voice 2 (the dotted quarter-eighth chords). Follow these instructions:

Click the dotted half note (F) in measure 8 to select it.

> I believe this next to be the step missed most often by people who have trouble with Voices. You must be in note entry mode to enter notes into Voice 2-4. ... Voice 1, for that matter, too :-)

Press N to activate note entry mode
Press Ctrl+Alt+2

> You'll see that the Voice 2 button activates on the toolbar.

Press 5, . (a period, or "full stop").

> This sets the note's duration to be a dotted quarter note.

Press A.

> The A will appear, but it might be in the wrong octave, Press Ctrl+Up or Ctrl+Down to move it up or down by whole octaves.

Then press Shift+C to add the C to a chord with the A.
Press 4 to set the duration to an eighth note.
Now press T. This will create the same chord (even if it's only a single note) tied to the previous chord, with the current duration.

The rests will fill in automagically.

Now, on to measure 7. There are actually TWO (slightly) unusual things you have to do here: 1) use two voices on a staff, and 2) create "cross-staff notation". You'll find the Handbook entry for this at https://handbook.musescore.org/idiomatic-notation/keyboard/cross-staff-….

First we'll create the second voice. To match the appearance of that measure on your original, I recommend putting the notes on the bass clef. Generally, you want notes with stems up to be Voice 1 (and 3, if necessary) and notes with stems down to be Voice 2 (and 4, if necessary). These voices will default to that orientation if both voices are present (but you can always reverse them!). So we're going to put all the "extra" notes--including the ones on the treble clef--into Voice 1 on the bass clef. In measure 7, you already have the dotted quarter notes (stems down) in Voice 2, so that's already good.

Click on the "whole-measure rest" to select it.

> Note that this is not a "whole rest". A whole rest will appear at beat one, the same place where a whole note would be placed. The whole-measure rest takes up a full measure, no matter the time signature.
> Note that, when selected the whole-measure rest is blue. That means that it is in Voice 1.

Press 4 to set the duration to eighth note.
Press 0 to enter a rest.
Press = (equals), D to enter an explicitly natural D.

> Again, use Ctrl+Up or Ctrl+Down if necessary to move the note to the correct octave.

Press =, Shift+E to add the natural E to a chord with the current note (the D).
Press A.
Press 0 to add the rest.
Press D.
Press C.

NOW, the correct notes are present but the three are not on the treble clef where they belong.

Press N to turn OFF note entry mode.
Click on either of the two notes in the chord. Then Shift+click on the A. This will select all three of the notes that are supposed to be on the treble clef.
Press Ctrl+Shift+Up.

This last is the keystroke to use the "cross-staff notation" I mentioned above. And with that, the notation is as you want it.

To get better used to using voices, read the Handbook section you looked at before, read my instructions again, and try to create a section like I put on your measures 14-16.