How can i save my work at 300 ppi in Lightroom ver. CC2020?

A digital image doesn't really have a PPI setting. Consider an image that is 6000 x 4000 pixels. It is that size regardless of whether you have designated the PPI to be 300 or 600 or 72 or whatever. You cannot change the resolution by adjusting the PPI. What you need to be concerned about is whether or not there are enough pixels to print the image at the size you want. For instance, consider that you want an image that is printed 8 x 10", and you want 300 PPI. Do the math:

8" @ 300 PPI (8 * 300) = 2400 pixels

10"  @ 300 PPI (10 * 300)) = 3000 Pixels

So for that image you need a file that is 2400 x 3000 pixels, and it doesn't matter what the PPI setting is set to. In the past I have had images printed the same size, 8 x 10" at a local lab, different PPI settings, specifically to compare to prove this test. One image set at 72 PPI, the other set at 300 PPI, image dimensions exactly the same, and the quality of the prints was identical. The PPI setting is irrelevant, the number of pixels IS the relevant number.