This is driving me insane. I installed Grav onto my server. No issues and theme shows fine. I tried installing the admin plugin but when I go to my url /admin I get a 404 error. I have already created a user but still get the 404 page.
My server is running nginx, I’ve searched Google x3 over and still cant find an answer that fixes this issue. 
Most probably the redirection is not configured correctly for grav in your server.
have a look at https://github.com/getgrav/grav/blob/develop/webserver-configs/nginx.conf for reference.
This is my current nginx.conf. Annoyingly enough, this would also be my first time using nginx also. I’ve only every used Apache until now.
# Server globals
user nginx;
worker_processes auto;
worker_rlimit_nofile 65535;
error_log /var/log/nginx/error.log crit;
pid /var/run/nginx.pid;
# Worker config
events {
worker_connections 1024;
use epoll;
multi_accept on;
}
http {
# Main settings
sendfile on;
tcp_nopush on;
tcp_nodelay on;
client_header_timeout 1m;
client_body_timeout 1m;
client_header_buffer_size 2k;
client_body_buffer_size 256k;
client_max_body_size 256m;
large_client_header_buffers 4 8k;
send_timeout 30;
keepalive_timeout 60 60;
reset_timedout_connection on;
server_tokens off;
server_name_in_redirect off;
server_names_hash_max_size 512;
server_names_hash_bucket_size 512;
# Log format
log_format main '$remote_addr - $remote_user [$time_local] $request '
'"$status" $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"';
log_format bytes '$body_bytes_sent';
#access_log /var/log/nginx/access.log main;
access_log off;
# Mime settings
include /etc/nginx/mime.types;
default_type application/octet-stream;
# Compression
gzip on;
gzip_comp_level 9;
gzip_min_length 512;
gzip_buffers 8 64k;
gzip_types text/plain text/css text/javascript text/js text/xml application/json application/javascript application/x-javascript application$
gzip_proxied any;
gzip_disable "MSIE [1-6]\.";
# Proxy settings
proxy_redirect off;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_pass_header Set-Cookie;
proxy_connect_timeout 90;
proxy_send_timeout 90;
proxy_read_timeout 90;
proxy_buffers 32 4k;
# Cloudflare https://www.cloudflare.com/ips
set_real_ip_from 103.21.244.0/22;
set_real_ip_from 103.22.200.0/22;
set_real_ip_from 103.31.4.0/22;
set_real_ip_from 104.16.0.0/12;
set_real_ip_from 108.162.192.0/18;
set_real_ip_from 131.0.72.0/22;
set_real_ip_from 141.101.64.0/18;
set_real_ip_from 162.158.0.0/15;
set_real_ip_from 172.64.0.0/13;
set_real_ip_from 173.245.48.0/20;
set_real_ip_from 188.114.96.0/20;
set_real_ip_from 190.93.240.0/20;
set_real_ip_from 197.234.240.0/22;
set_real_ip_from 198.41.128.0/17;
#set_real_ip_from 2400:cb00::/32;
#set_real_ip_from 2606:4700::/32;
#set_real_ip_from 2803:f800::/32;
#set_real_ip_from 2405:b500::/32;
#set_real_ip_from 2405:8100::/32;
#set_real_ip_from 2c0f:f248::/32;
#set_real_ip_from 2a06:98c0::/29;
real_ip_header CF-Connecting-IP;
# SSL PCI Compliance
ssl_session_cache shared:SSL:10m;
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
ssl_prefer_server_ciphers on;
ssl_ciphers "ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA3$
# Error pages
error_page 403 /error/403.html;
error_page 404 /error/404.html;
error_page 502 503 504 /error/50x.html;
# Cache settings
proxy_cache_path /var/cache/nginx levels=2 keys_zone=cache:10m inactive=60m max_size=1024m;
proxy_cache_key "$host$request_uri $cookie_user";
proxy_temp_path /var/cache/nginx/temp;
proxy_ignore_headers Expires Cache-Control;
proxy_cache_use_stale error timeout invalid_header http_502;
proxy_cache_valid any 1d;
# Cache bypass
map $http_cookie $no_cache {
default 0;
~SESS 1;
~wordpress_logged_in 1;
}
# File cache settings
open_file_cache max=10000 inactive=30s;
open_file_cache_valid 60s;
open_file_cache_min_uses 2;
open_file_cache_errors off;
# Wildcard include
include /etc/nginx/conf.d/*.conf;
}The Nginx conf is definitely helpful, but would you be able to share the server block for the website (you can prune it of identifying things).
The redirect mentioned would have been placed in your server block (most likely included from the last line of the nginx.conf.)
include /etc/nginx/conf.d/*.conf;
The sample Nginx server block can be found here.