there's a difference between importing a 1099-B and trying to uploading and attaching it. if you are importing then the issue is at the broker's end. you need to contact them. Turbotax will not accept any .pdf attachments to an e- filed return. the issue is that when transmitted the tax return is converted to a binary file and since 1099-B's differ a little from broker to broker there's no way for TurboTax to convert it. even if it could since the 1099-B has no official IRS sequence number the return would be rejected because the IRS computers would not understand the binary.
if you have a voluminous number of transactions (like over 2 or 3,000 depending on TT version) you can enter summary totals on the 1099-B worksheet. on that worksheet, you also need to enter code M for each line. then using form 8453 with the box for 8949 marked (read instructions) you can mail it and the 1099-B to the IRS.
https://www.irs.gov/pub/irs-pdf/f8453.pdf