You're also missing a closing bracket in your `body` from the looks of it

const D1URL = 'https://api.cloudflare.com/client/v4/accounts/4f6d1970bc967b2c2a90b643bbb5ac94/d1/database/0e35121-f3cc-4e24-b5c1-4d758860a69d/query';

const insertStatement = `INSERT INTO table (c1, c2, c3) VALUES (v1, v2, v3)`;

const options = {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      Authorization: 'Bearer GAQjclxUayPOaHhn5Nx1CGdyN2Pt08COkmFTFBjm'
    },
    body: JSON.stringify({
      sql: insertStatement
    })
  }

fetch(D1URL, options)
    .then( res => res.json())
    .then( data => console.log(data))

const D1URL = 'https://api.cloudflare.com/client/v4/accounts/4f6d1970bc967b2c2a90b643bbb5ac94/d1/database/0e35121-f3cc-4e24-b5c1-4d758860a69d/query';

const insertStatement = `INSERT INTO table (c1, c2, c3) VALUES (v1, v2, v3)`;

const options = {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      Authorization: 'Bearer GAQjclxUayPOaHhn5Nx1CGdyN2Pt08COkmFTFBjm'
    },
    body: JSON.stringify({
      sql: insertStatement
    })
  }

fetch(D1URL, options)
    .then( res => res.json())
    .then( data => console.log(data))

"result": {
"success": true,
"meta": {
"served_by": "v3-prod",
"duration": 0.1743,
"changes": 0,
"last_row_id": 0,
"changed_db": false,
"size_after": 25100288,
"rows_read": 1804,
"rows_written": 0
},
"results": [
{
"COUNT(*)": 1804
}
]

"result": {
"success": true,
"meta": {
"served_by": "v3-prod",
"duration": 0.1743,
"changes": 0,
"last_row_id": 0,
"changed_db": false,
"size_after": 25100288,
"rows_read": 1804,
"rows_written": 0
},
"results": [
{
"COUNT(*)": 1804
}
]