The argument "api_key" is required, but was not set.

[thevops]

There is comment that UPTIMEROBOT_API_KEY can be set by environment variable

provider "uptimerobot" {
  api_key = "[YOUR MAIN API KEY]" # or pass via environment variable UPTIMEROBOT_API_KEY
}

but in that case terraform validate raises error:

Error: Missing required argument

The argument "api_key" is required, but was not set.