I'm having a problem when I try to get into macOS Utilities. When I hold CMD + R it shows me a prohibited symbol. I'm trying to downgrade my MacBook because its running very slow. Currently I have Mojave installed and I want to donwgrade it to High Sierra. Before I installed Mojave everything was fine. I could go to the utilities without any problem. it happened after I installed it and can't access the utilities.
About my Mac: macOS Mojave Version 10.14.6 Macbook (13-inch, Aluminum, Late 2008) Processor 2 GHz Intel Core 2 Duo Memory 4 GB 1067 Mhz DDR3 Graphics NVIDIA GeForce 9400M 256 MB