This thread is related to your problem:
http://forum.coppermine-gallery.net/index.php?topic=25197.0.
Basically, Coppermine is looking for two tags that define the report_file_button block (the tags are <!-- BEGIN report_file_button --> and <!-- END report_file_button -->) so it can place the report_file_button block in the correct place when it outputs a web page. If you don't override the variable $template_img_navbar in your theme.php, then the default $template_img_navbar is used which includes these tags and the default report_file_button block. If you override $template_img_navbar but don't include these 2 tags, then Coppermine chokes on the vacuum you created. As mentioned in the thread above, if you don't want this block on your web pages, just leave the place between the 2 tags blank; Coppermine will see the tags and realize you don't want any report_file_button block.
The sample theme has a theme.php which includes every variable and function you can override in your theme. So whenever you have a theme question, check out the sample theme (it is not meant to be used as a display theme - only an example theme for you to copy from). Search for the part in question, in this case <!-- BEGIN report_file_button --> since your error message mentions that. Then compare the section in the sample theme to your theme.php. Understanding what Coppermine is looking for (well discussed in the thread above) is the baseline you need to solve theme problems. In your case, you probably have $template_img_navbar in your theme.php but are missing the report_file_button block. I would copy the whole $template_img_navbar definition from the sample theme into your theme.php, then modify it according to your current $template_img_navbar definition. After modifying it, remove the previous $template_img_navbar definition so you only have one in your theme.php.
Here's the relevant code from the sample theme to your situation:
// HTML template for the image navigation bar
$template_img_navbar = <<<EOT
<tr>
<td align="center" valign="middle" class="navmenu" width="48px"><a name="top_display_media"></a>
<a href="{THUMB_TGT}" class="navmenu_pic" title="{THUMB_TITLE}"><img src="{LOCATION}images/thumbnails.gif" align="middle" border="0px" alt="{THUMB_TITLE}" /></a>
</td>
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="javascript:;" class="navmenu_pic" onclick="blocking('picinfo','yes', 'block'); return false;" title="{PIC_INFO_TITLE}"><img src="{LOCATION}images/info.gif" border="0px" align="middle" alt="{PIC_INFO_TITLE}" /></a>
</td>
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{SLIDESHOW_TGT}" class="navmenu_pic" title="{SLIDESHOW_TITLE}"><img src="{LOCATION}images/slideshow.gif" border="0px" align="middle" alt="{SLIDESHOW_TITLE}" /></a>
</td>
<td align="center" valign="middle" class="navmenu" width="100%">
{PIC_POS}
</td>
<!-- BEGIN report_file_button -->
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{REPORT_TGT}" class="navmenu_pic" title="{REPORT_TITLE}"><img src="{LOCATION}images/report.gif" border="0" align="middle" alt="{REPORT_TITLE}" /></a>
</td>
<!-- END report_file_button -->
<!-- BEGIN ecard_button -->
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{ECARD_TGT}" class="navmenu_pic" title="{ECARD_TITLE}"><img src="{LOCATION}images/ecard.gif" border="0px" align="middle" alt="{ECARD_TITLE}" /></a>
</td>
<!-- END ecard_button -->
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{PREV_TGT}" class="navmenu_pic" title="{PREV_TITLE}"><img src="{LOCATION}images/prev.gif" border="0px" align="middle" alt="{PREV_TITLE}" /></a>
</td>
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{NEXT_TGT}" class="navmenu_pic" title="{NEXT_TITLE}"><img src="{LOCATION}images/next.gif" border="0px" align="middle" alt="{NEXT_TITLE}" /></a>
</td>
</tr>
EOT;
Finally, you might run into more issues related to using an old theme in the current version of Coppermine. It is strongly recommended that you read the "upgrading your theme" section in the Coppermine documentation (conveniently located in your Coppermine installation, linked from the "Documentation" button in your admin toolbar).
Teach a person to fish . . .