Generally speaking, when you're interacting with any command line interface, there are two things you should definitely avoid to mimize chances of problems:

• Non ASCII characters ^[1]
• Spaces

You've got both of them here (and multiple occurrences of each) unfortunately. Luckily, your windows USERNAME is perfect (no spaces, no non-ASCII characters) so worst case you can just move things around.

Two things you can try:

cd to the directory and run

cd "C:/Users/Neon_Demiurge/Documents/ΗΜΜΥ/Πέμπτο έτος(2022 - 2023)/Διπλωματική(2023)/Energy modeling/my models/my model 1/my model 1/"
C:\openstudioapplication-1.5.0\bin>openstudio run --workflow workflow.osw

Escape the path with spaces

Add quotes around the path with space (or let autocomplete via the TAB key escape it inline)

C:\openstudio-3.5.1\bin>openstudio run --workflow "C:/Users/Neon_Demiurge/Documents/ΗΜΜΥ/Πέμπτο έτος(2022 - 2023)/Διπλωματική(2023)/Energy modeling/my models/my model 1/my model 1/workflow.osw"

 Last resort

Just use a safer path on disk (and rename your model too).

eg C:/Users/Neon_Demiurge/Documents/ΗΜΜΥ/Energy_Modeling/my_model1/model.osm

Notes:

^[1]I know using only ASCII characters is annoying especially if your native language uses them (I'm French and learned this the hard way). But that's what it is, the vast majority of systems ever built have some amount of trouble with non-ASCII characters or encoding generally, and this is still problematic today as encoding is different on Windows cmd.exe/pwsh.exe (latin-1, and UTF-16 strings) versus Unix terminals (UTF-8).