The error message appears mostly self-explanatory as to which rate limit you are running into. The text "You can only post once every 90 minutes." is quite clear that you can only post one question every 90 minutes.

However, the rate limit is probably more expansive than you think it is. The parts you may not be taking into account are that the question asking rate limits are based on both account (across all Stack Exchange sites) and the IP address used to attempt to post the question. So, presumably, you, or someone on the IP address you are using, has already posted a question somewhere on the Stack Exchange Network within the prior 90 minutes.

See: answer to: "The Complete Rate-Limiting Guide" for a (mostly) complete description of the rate limits which Stack Exchange applies.

The error message appears mostly self-explanatory as to which rate limit you are running into. The text "You can only post once every 90 minutes." is quite clear that you can only post one question every 90 minutes.

However, the rate limit is probably more expansive than you think it is. The parts you may not be taking into account are that the question asking rate limits are based on both the account and the IP address used to attempt to post the question and take into account questions posted on all Stack Exchange sites. So, presumably, you, or someone on the IP address you are using, has already posted a question somewhere on the Stack Exchange Network within the prior 90 minutes. To be clear, that means you could be blocked by this rate limit as a result of the actions of completely different people who happen to share your IP address. Depending on your networking setup, quite a few people may be sharing your public IP address.

See: answer to: "The Complete Rate-Limiting Guide" for a (mostly) complete description of the rate limits which Stack Exchange applies.