I can think of two possible solutions:
- Run the SMAPI installer as an administrator.
If that doesn’t work, try this:
- Go to Program Files (x86) > Steam > steamapps > common.
- Right-click on the Stardew Valley folder and select Properties.
- Navigate to the Security tab, then click Edit under the list of group or user names.
- For each group or user, check the Allow box for Full Control and apply the changes.