So when ever y try to place a sentry on the server me and my friends are at, it crashes.

Are you using the latest version of SecurityCraft (v1.8.18.1 for Minecraft 1.14.4/1.15.2 and v1.8.18.2 for Minecraft 1.12.2)? If not, please update and check if the crash is still happening. If so, post the crash-report.
When reporting crashes, always include the crash-report. Without it, us developers can do next to nothing in order to try and fix the crash. You can find it in the crash-reports folder, in the same place where the mods folder is.

Closing this as there is not enough information. If you can provide it, feel free to comment.