One of the main reasons why this error appears is because the servers are overloaded. So there is nothing you can do and you will have to wait until the developers release an update or patch to fix this problem.
Some users in the EOT community have suggested reinstalling the Battlestate Game launcher, which could perhaps work if it's a new patch, but the download itself is unlikely to fix it.
For now this is all there is on how to fix error on Post in Escape From Tarkov, so we will have to wait for the developers to address this issue.