Netplan Error in Network Definition Expected Mapping

a netplan configuration is base on yaml, when looking your configuration. i think the indentation is error because its have 3 space. maybe you can try this

# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config:disabled}
network:
  version: 2
  ethernets:
    enp0s3:expected mapping
      addresses: [192.168.0.183/24]
      gateway4: 192.168.0.250
      dhcp4: false
      nameservers: 
        addresses: [8.8.8.8,8.8.4.4]
      optional: true

I always configure for a static ip the "dhcp" with the value of "no" like this dhcp4: no

See an example below

network:
    ethernets:
        enp0s3:
            dhcp4: no
            dhcp6: no
            addresses: [10.4.1.174/21]
            gateway4: 10.5.1.6
            nameservers:
                addresses: [8.8.4.4,8.8.8.8]
            routes:
                - to: 192.168.0/24
                  via: 10.5.0.1
                  metric: 100
    version: 2

My first time coming across this new way of networking and the "expected mapping" error. It's a nice one.

I got a static IP working by editing etc/netplan/50-cloud-init.yaml but found that it was SUPER picky about the layout (spaces).

I used the examples given at

By going to the line BEFORE the line mentioned in the error (given when it didn't work) and pressing "return", a new line and the correct amount of indent was automatically inserted.

Restated... I had an error reported in line 12, so I went to the end of line 11 and pressed "return". A carriage return AND the (presumably) correct amount of indentation was automatically inserted.

If the error was at line 1 (as for the OP) I can only guess that the syntax is wrong.

I learned that if the indents are not perfect, it does not work. Even though I went off another example, the example I found was incorrect. Here's the contents of 00-installer-config.yaml that worked for me in case it helps someone else later:

network:
    ethernets:
        enol:
           dhcp4: no
           addresses: [192.168.0.45/24]
           gateway4: 192.168.0.1
           nameservers:
               addresses: [8.8.8.8, 8.8.8.4]
    version: 2

Here is a screenshot of 00-installer-config.yaml.

I tried several formats

Ubuntu 20 (netplan apply command) only accepted config after I configured exact spaces before each line:

# This is the network config written by 'subiquity'

network:
 version: 2
 renderer: networkd
 ethernets:
  ens18:
   addresses: [172.16.1.95/24]
   gateway4: 172.16.1.1
  nameservers:
   addresses: [8.8.8.8/32, 8.8.4.4/32]

After that, error went away and I was able to connect to 172.16.1.95 server via SSH