silence apareance in third voice

Please attach a score showing what you mean. "Silence Figur" is a rest, isn't it? Voice 1 rests cannot get deleted, and that much is by design. You can make them invisible though.

The silence (rest in English) is in voice 1. You cannot delete voice 1 rests. You either need to exchange voice 1 with one of the others (probably 3) or select the rest and press V to make it invisible. It will appear gray if you have Show Invisible checked in the View menu.

You should use voice 1 in the bottom staff, not 3. Each staff has up to 4 voices, and they count per staff, not per instrument. See https://musescore.org/en/handbook/voices and check the attached corrected score.
Bertter to use the forum for support requests, not the issue tracker.

What Jojo was telling you is that the left hand of your piano piece has everything entered into voice 3, forcing rests to appear in voice 1. You can have 4 voice in the right hand AND 4 voices in the left hand. Always start each hand with voice 1 and the only extra rests you will see are the ones that fill out the rest of a measure or that you enter yourself.

Both hands start with voice and optionally have voices 2-4
Only ever use voice 3 and 4, if 1 and 2 are used already, in the same staff
Read the manual I linked to above and apply the sentence about SATB to piano.