It looks like the error you're getting is due to lack of write permissions to the reshade.ini file. To fix this, you need to do the following steps:
Find the reshade.ini file in the game folder where you installed ReShade. This should be the same folder where you have the FiveM.exe file.
Right click on the reshade.ini file and select Properties.
Click the Security tab and then click Edit.
On the Permissions window, click the Add button to open the Select User or Group window.
Enter People in the Enter object name to select box and click Check names. Then click OK.
On the Permissions window, select Everyone and check the Full Control box under Allow. Then click OK.
Click OK again to close the Properties window.
