Problem
You can't add materials to Pricebook because vendors are either missing or appear but you can't choose them as the Primary vendor.
Solution
Pricebook requires a vendor to be active before it can be assigned as the material's primary vendor.
Activate and select a primary vendor
Go to the navigation bar and click Pricebook.
In the side panel, click Materials.
Click +Add Material.
In the Replenishment vendor section, click Show/Hide Inactive Vendors.

Click the Active checkbox next to the vendor name.

After marking the vendor as Active, select it as the Primary vendor.
Note: If only one vendor is active, it will be automatically selected as the Primary vendor.
When finished adding the material details, click Save.
Enable replenishment vendor
If vendors are missing from the list entirely:
Go to the top toolbar and click Settings
.From the side menu, select Purchasing or Inventory > Vendors.
On the Vendors screen that opens, select the vendor you want to edit.
Tip: Use the Search field to find the vendor you want.
On the vendor profile screen that opens, click Edit.

In the Vendor Details section, select the Replenishment Vendor option

When finished, click Save.
The vendor should now appear as an option in the vendor list.
For more, see Set up vendor list and primary vendor for pricebook items.