Hi @sgkiourtzis! Welcome!
It's hard to know quite what's going wrong here without seeing the code you used to create the DOKIMASTIKO6 data frame. It looks like the data got transposed somewhere along the way. If you look in your Environment pane you'll see that DOKIMASTIKO6 has "2 obs. of 504 variables" — in other words, 2 rows and 504 columns. It seems like it ought to have 504 rows and 2 columns, instead?
The best way to get an answer is to turn this question into a small reproducible example, so that other people can see the code and run it for themselves. Start by reading the tips here: FAQ: Tips for writing R-related questions
Making reproducible examples for data import questions can be tricky, because the code depends on an external file (the data you imported). If the data is shareable, you can post the file somewhere that people here can access, like Google Drive or GitHub. If it's not shareable, then you'll need to create an example file that shows the same problem as your real file when imported using the same code.
If you get stuck on a reproducible example, please at least try to post your import code, following all the tips in the above link! 